ci: fix size impact script Node.js version

[guybedford]

Fixes up the Node.js version to work with the current build scripts.

[github-actions]

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Error: Error while trying to generate a snapshot for main.

Error: Command failed: npm run-script build
file:///home/runner/work/systemjs/systemjs//[cm]:7
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:7
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:5
  const { code, map, assets } = await ncc(path.resolve(process.env.DEP), {
                                ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
file:///home/runner/work/systemjs/systemjs//[cm]:4
  const bundle = await rollup({
                 ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:122:18)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async link (internal/modules/esm/module_job.js:42:21)
Unable to complete all tasks.
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! systemjs@6.12.1 build: `chomp build`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the systemjs@6.12.1 build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/runner/.npm/_logs/2022-07-24T09_48_14_990Z-debug.log

    at ChildProcess.exithandler (child_process.js:308:12)
    at ChildProcess.emit (events.js:314:20)
    at maybeClose (internal/child_process.js:1051:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:287:5)

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_{Generated by file size impact during size-impact#2726801878}