Enables postinstall scripts for pnpm (#6902)

## What's the problem this PR addresses?

It came to me that since pnpm no longer runs postinstall scripts by
default, the benchmark was a little skewed since Yarn (and npm) do. The
benchmarks are intended to compare apples to apples, so either
postinstalls should be disabled in Yarn / npm as well, or they should be
enabled in pnpm. I think it makes more sense to enable them, as it can
uncover bugs that would otherwise be hidden away.

## How did you fix it?

Enabled the postinstall scripts in the package manager benchmarks.

To be fair it probably doesn't change much, if anything - the full cold
Gatsby benchmark shows around 2 seconds of removed install time around
Feb 3rd 2025 which I think is caused by the lack of postinstall,
although I'm not certain, but that's about it. Pnpm remains really fast,
kudos!

<img width="145" height="93" alt="Screenshot 2025-09-18 at 10 43 02"
src="https://github.com/user-attachments/assets/3f9a362a-5c71-41c1-99b2-b2bc930ca796"
/>

Curiously the same change occurs in the "Recurrent calls" benchmark
which probably shouldn't need to run postinstall scripts. Does pnpm have
a bug causing it to rebuild postinstall scripts when they aren't needed?

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Author: arcanis

scripts/bench-run.sh

@@ -60,6 +60,8 @@ setup-yarn2-pnpm() {
 setup-pnpm() {
   >> "$BENCH_DIR/.npmrc" echo \
     "strict-peer-dependencies=false"
+  >> "$BENCH_DIR/pnpm-workspace.yaml" echo \
+    "dangerouslyAllowAllBuilds: true"
 }
 
 case $PACKAGE_MANAGER in