Fix generateBuildId type that can be async function

[xnuk]

The current documentation ensures it can be async function, so this PR fixes the TypeScript type of it.

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	xnuk/next.js patch-1	Change
buildDuration	12.5s	12.7s	⚠️ +221ms
buildDurationCached	3s	2.9s	-98ms
nodeModulesSize	49.1 MB	49.1 MB	⚠️ +100 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	xnuk/next.js patch-1	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.273	2.271	0
/ avg req/sec	1100.11	1100.96	+0.85
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.294	1.269	-0.03
/error-in-render avg req/sec	1932.44	1970.2	+37.76

Client Bundles (main, webpack, commons)

	vercel/next.js canary	xnuk/next.js patch-1	Change
745.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	23.1 kB	23.1 kB	✓
webpack-HASH.js gzip	1.5 kB	1.5 kB	✓
Overall change	67 kB	67 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	xnuk/next.js patch-1	Change
polyfills-HASH.js gzip	31.1 kB	31.1 kB	✓
Overall change	31.1 kB	31.1 kB	✓

Client Pages

	vercel/next.js canary	xnuk/next.js patch-1	Change
_app-HASH.js gzip	980 B	980 B	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	329 B	329 B	✓
dynamic-HASH.js gzip	2.52 kB	2.52 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	904 B	904 B	✓
image-HASH.js gzip	4.13 kB	4.13 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	319 B	319 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	12.8 kB	12.8 kB	✓

Client Build Manifests

	vercel/next.js canary	xnuk/next.js patch-1	Change
_buildManifest.js gzip	491 B	491 B	✓
Overall change	491 B	491 B	✓

Rendered Page Sizes

	vercel/next.js canary	xnuk/next.js patch-1	Change
index.html gzip	531 B	531 B	✓
link.html gzip	543 B	543 B	✓
withRouter.html gzip	524 B	524 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Webpack 4 Mode

General Overall increase ⚠️

	vercel/next.js canary	xnuk/next.js patch-1	Change
buildDuration	10.6s	10.5s	-88ms
buildDurationCached	4.2s	4.1s	-37ms
nodeModulesSize	49.1 MB	49.1 MB	⚠️ +100 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	xnuk/next.js patch-1	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.33	2.33	✓
/ avg req/sec	1072.85	1072.96	+0.11
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.277	1.277	✓
/error-in-render avg req/sec	1957.11	1957.96	+0.85

Client Bundles (main, webpack, commons)

	vercel/next.js canary	xnuk/next.js patch-1	Change
17.HASH.js gzip	185 B	185 B	✓
677f882d2ed8..HASH.js gzip	14 kB	14 kB	✓
framework.HASH.js gzip	41.9 kB	41.9 kB	✓
main-HASH.js gzip	10.6 kB	10.6 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	67.8 kB	67.8 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	xnuk/next.js patch-1	Change
polyfills-HASH.js gzip	31.3 kB	31.3 kB	✓
Overall change	31.3 kB	31.3 kB	✓

Client Pages

	vercel/next.js canary	xnuk/next.js patch-1	Change
_app-HASH.js gzip	965 B	965 B	✓
_error-HASH.js gzip	3.71 kB	3.71 kB	✓
amp-HASH.js gzip	552 B	552 B	✓
css-HASH.js gzip	333 B	333 B	✓
dynamic-HASH.js gzip	2.71 kB	2.71 kB	✓
head-HASH.js gzip	2.97 kB	2.97 kB	✓
hooks-HASH.js gzip	911 B	911 B	✓
index-HASH.js gzip	231 B	231 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	298 B	298 B	✓
script-HASH.js gzip	2.95 kB	2.95 kB	✓
withRouter-HASH.js gzip	294 B	294 B	✓
e025d2764813..52f.css gzip	125 B	125 B	✓
Overall change	17.7 kB	17.7 kB	✓

Client Build Manifests

	vercel/next.js canary	xnuk/next.js patch-1	Change
_buildManifest.js gzip	498 B	498 B	✓
Overall change	498 B	498 B	✓

Rendered Page Sizes

	vercel/next.js canary	xnuk/next.js patch-1	Change
index.html gzip	576 B	576 B	✓
link.html gzip	590 B	590 B	✓
withRouter.html gzip	570 B	570 B	✓
Overall change	1.74 kB	1.74 kB	✓

Commit: f2fc3d3