simplify creation of dist/env.d.cts (#143)

* chore: skip generating lockfiles for test fixtures

* chore: simplify creation of dist/env.d.cts

This changes how `dist/env.d.cts` gets generated. Instead of re-running `tsc` with a unique config, we can just copy `dist/env.d.ts`.

We could pull in something like `ncp` or whatever else to do the copy, if we hate the one-liner here.

package.json

@@ -35,7 +35,7 @@
     "url": "https://github.com/sponsors/nzakas"
   },
   "scripts": {
-    "build": "rollup -c && tsc && tsc -p tsconfig.cjs.json",
+    "build": "rollup -c && tsc && node -e \"fs.copyFileSync('./dist/env.d.ts', './dist/env.d.cts')\"",
     "prepare": "npm run build",
     "lint": "eslint src/ tests/",
     "pretest": "npm run build",