ci: only run release commit check on canary releases

[ztanner]

We don't want to perform this check for stable releases since those are never initiated by the cron task, as otherwise it would fail when going from canary -> stable as no commits would land in-between.

Closes NEXT-1836

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
buildDuration	10.5s	10.8s	⚠️ +255ms
buildDurationCached	6s	6s	N/A
nodeModulesSize	199 MB	199 MB	✓
nextStartRea..uration (ms)	425ms	431ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
170-HASH.js gzip	26.7 kB	26.7 kB	N/A
199.HASH.js gzip	181 B	185 B	N/A
3f784ff6-HASH.js gzip	53.3 kB	53.3 kB	✓
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	242 B	241 B	N/A
main-HASH.js gzip	31.7 kB	31.6 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	N/A
Overall change	98.5 kB	98.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
_app-HASH.js gzip	195 B	194 B	N/A
_error-HASH.js gzip	183 B	182 B	N/A
amp-HASH.js gzip	501 B	501 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.5 kB	2.5 kB	N/A
edge-ssr-HASH.js gzip	255 B	255 B	✓
head-HASH.js gzip	349 B	350 B	N/A
hooks-HASH.js gzip	368 B	369 B	N/A
image-HASH.js gzip	4.27 kB	4.27 kB	N/A
index-HASH.js gzip	255 B	256 B	N/A
link-HASH.js gzip	2.6 kB	2.6 kB	N/A
routerDirect..HASH.js gzip	311 B	309 B	N/A
script-HASH.js gzip	384 B	384 B	✓
withRouter-HASH.js gzip	307 B	306 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	1.57 kB	1.57 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
_buildManifest.js gzip	483 B	483 B	✓
Overall change	483 B	483 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
index.html gzip	529 B	529 B	✓
link.html gzip	542 B	542 B	✓
withRouter.html gzip	524 B	525 B	N/A
Overall change	1.07 kB	1.07 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
edge-ssr.js gzip	93.5 kB	93.5 kB	N/A
page.js gzip	146 kB	146 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
middleware-b..fest.js gzip	622 B	625 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	37.4 kB	37.4 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	2.07 kB	2.07 kB	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js fix/release-skip	Change
app-page-exp...dev.js gzip	168 kB	168 kB	✓
app-page-exp..prod.js gzip	93.9 kB	93.9 kB	✓
app-page-tur..prod.js gzip	94.7 kB	94.7 kB	✓
app-page-tur..prod.js gzip	89.2 kB	89.2 kB	✓
app-page.run...dev.js gzip	138 kB	138 kB	✓
app-page.run..prod.js gzip	88.6 kB	88.6 kB	✓
app-route-ex...dev.js gzip	24 kB	24 kB	✓
app-route-ex..prod.js gzip	16.7 kB	16.7 kB	✓
app-route-tu..prod.js gzip	16.7 kB	16.7 kB	✓
app-route-tu..prod.js gzip	16.3 kB	16.3 kB	✓
app-route.ru...dev.js gzip	23.4 kB	23.4 kB	✓
app-route.ru..prod.js gzip	16.3 kB	16.3 kB	✓
pages-api-tu..prod.js gzip	9.37 kB	9.37 kB	✓
pages-api.ru...dev.js gzip	9.64 kB	9.64 kB	✓
pages-api.ru..prod.js gzip	9.37 kB	9.37 kB	✓
pages-turbo...prod.js gzip	21.9 kB	21.9 kB	✓
pages.runtim...dev.js gzip	22.6 kB	22.6 kB	✓
pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	49.4 kB	49.4 kB	✓
Overall change	930 kB	930 kB	✓

Commit: 458aff7