Update test config to only install pnpm when needed

[ijjk]

Only one test in the integration folder leverages pnpm so this ensures we only install pnpm for that specific case.

x-ref: https://github.com/vercel/next.js/runs/6612689318?check_suite_focus=true

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
buildDuration	15.5s	15.9s	⚠️ +389ms
buildDurationCached	6.3s	6.2s	-41ms
nodeModulesSize	494 MB	494 MB	✓

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.825	3.816	-0.01
/ avg req/sec	653.62	655.11	+1.49
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.315	1.348	⚠️ +0.03
/error-in-render avg req/sec	1900.86	1855.1	⚠️ -45.76

Client Bundles (main, webpack)

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
925.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	29.2 kB	29.2 kB	✓
webpack-HASH.js gzip	1.54 kB	1.54 kB	✓
Overall change	73 kB	73 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
_app-HASH.js gzip	1.36 kB	1.36 kB	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	308 B	308 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.71 kB	2.71 kB	✓
head-HASH.js gzip	359 B	359 B	✓
hooks-HASH.js gzip	920 B	920 B	✓
image-HASH.js gzip	5.74 kB	5.74 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.65 kB	2.65 kB	✓
routerDirect..HASH.js gzip	320 B	320 B	✓
script-HASH.js gzip	391 B	391 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	16 kB	16 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
index.html gzip	533 B	533 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

---

Default Build with SWC (Decrease detected ✓)

General

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
buildDuration	17.9s	17.9s	-27ms
buildDurationCached	6.2s	6.3s	⚠️ +19ms
nodeModulesSize	494 MB	494 MB	✓

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.835	3.821	-0.01
/ avg req/sec	651.94	654.22	+2.28
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.314	1.345	⚠️ +0.03
/error-in-render avg req/sec	1902.62	1858.38	⚠️ -44.24

Client Bundles (main, webpack)

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
925.HASH.js gzip	178 B	178 B	✓
framework-HASH.js gzip	42.7 kB	42.7 kB	✓
main-HASH.js gzip	29.7 kB	29.7 kB	✓
webpack-HASH.js gzip	1.54 kB	1.54 kB	✓
Overall change	74 kB	74 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	179 B	179 B	✓
amp-HASH.js gzip	311 B	311 B	✓
css-HASH.js gzip	324 B	324 B	✓
dynamic-HASH.js gzip	2.9 kB	2.9 kB	✓
head-HASH.js gzip	357 B	357 B	✓
hooks-HASH.js gzip	920 B	920 B	✓
image-HASH.js gzip	5.84 kB	5.84 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	2.78 kB	2.78 kB	✓
routerDirect..HASH.js gzip	322 B	322 B	✓
script-HASH.js gzip	392 B	392 B	✓
withRouter-HASH.js gzip	317 B	317 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	16.4 kB	16.4 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
_buildManifest.js gzip	458 B	458 B	✓
Overall change	458 B	458 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js remove/extra-pnpm-step	Change
index.html gzip	533 B	533 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	529 B	529 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: dcc3509