Add process.env.__NEXT_PPR to build config

[acdlite]

Adds process.env.__NEXT_PPR as static flag that it gets inlined by DefinePlugin during build.

This matches what we do for other experiments, but I do wonder, why do we bother with process.env at all instead of a global __NEXT_PPR symbol? I'll leave that for later discussion.

Closes NEXT-1792

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
buildDuration	10.7s	10.6s	N/A
buildDurationCached	7s	6.1s	N/A
nodeModulesSize	199 MB	199 MB	⚠️ +680 B
nextStartRea..uration (ms)	429ms	423ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
199-HASH.js gzip	30.7 kB	30.7 kB	N/A
3f784ff6-HASH.js gzip	53.3 kB	53.3 kB	✓
494.HASH.js gzip	180 B	181 B	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	243 B	239 B	N/A
main-HASH.js gzip	31.7 kB	31.7 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	100 kB	100 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
_app-HASH.js gzip	194 B	195 B	N/A
_error-HASH.js gzip	182 B	181 B	N/A
amp-HASH.js gzip	501 B	503 B	N/A
css-HASH.js gzip	322 B	323 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.5 kB	✓
edge-ssr-HASH.js gzip	253 B	255 B	N/A
head-HASH.js gzip	348 B	347 B	N/A
hooks-HASH.js gzip	369 B	368 B	N/A
image-HASH.js gzip	4.27 kB	4.27 kB	N/A
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.61 kB	2.6 kB	N/A
routerDirect..HASH.js gzip	311 B	311 B	✓
script-HASH.js gzip	384 B	383 B	N/A
withRouter-HASH.js gzip	307 B	308 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.17 kB	3.17 kB	✓

Client Build Manifests

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
_buildManifest.js gzip	484 B	483 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
index.html gzip	528 B	527 B	N/A
link.html gzip	538 B	540 B	N/A
withRouter.html gzip	524 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
edge-ssr.js gzip	92.6 kB	92.7 kB	N/A
page.js gzip	145 kB	145 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
middleware-b..fest.js gzip	627 B	626 B	N/A
middleware-r..fest.js gzip	150 B	151 B	N/A
middleware.js gzip	35.7 kB	35.7 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	1.92 kB	1.92 kB	✓

Next Runtimes

	vercel/next.js canary	acdlite/next.js add-static-ppr-flag	Change
app-page-exp...dev.js gzip	168 kB	168 kB	✓
app-page-exp..prod.js gzip	93.8 kB	93.8 kB	✓
app-page-tur..prod.js gzip	94.5 kB	94.5 kB	✓
app-page-tur..prod.js gzip	89.1 kB	89.1 kB	✓
app-page.run...dev.js gzip	138 kB	138 kB	✓
app-page.run..prod.js gzip	88.4 kB	88.4 kB	✓
app-route-ex...dev.js gzip	24.2 kB	24.2 kB	✓
app-route-ex..prod.js gzip	16.8 kB	16.8 kB	✓
app-route-tu..prod.js gzip	16.9 kB	16.9 kB	✓
app-route-tu..prod.js gzip	16.4 kB	16.4 kB	✓
app-route.ru...dev.js gzip	23.6 kB	23.6 kB	✓
app-route.ru..prod.js gzip	16.4 kB	16.4 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	49.3 kB	49.3 kB	✓
Overall change	930 kB	930 kB	✓

Commit: 514dc75