Add pnpm to api -> api-go sync Github Actions runner

[long-nt-tran]

What changed?

pnpm was recently added to the Makefile target for nexus-rpc-gen-install, which runs as part of the GHA that syncs protos from api to api-go via this command: make install update-proto test

We also need to add pnpm to here, otherwise the GHA runner will fail, i.e., https://github.com/temporalio/api-go/actions/runs/26116994851/job/76808739969:

Run make install update-proto test
  make install update-proto test
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
Install nexus-rpc-gen from main...
Cloning into '/home/runner/.cache/nexus-rpc-gen'...
/bin/sh: 1: pnpm: not found
make: *** [Makefile:133: nexus-rpc-gen-install] Error 127

Why?

So GHA runner works.

How did you test it?

Manual GHA kickoff from my branch by running:

$ gh workflow run update-proto.yml \
    --repo temporalio/api-go \
    --ref long-nt-tran/add-pnpm-gha-runner \
    --field branch=long-nt-tran/add-pnpm-gha-runner \
    --field commit_author="long-nt-tran" \
    --field commit_author_email="long.tran@temporal.io" \
    --field commit_message="Manual run to verify pnpm setup in GHA works"
✓ Created workflow_dispatch event for update-proto.yml at long-nt-tran/add-pnpm-gha-runner
https://github.com/temporalio/api-go/actions/runs/26121571272

To see the created workflow run, try: gh run view 26121571272
To see runs for this workflow, try: gh run list --workflow="update-proto.yml"

GHA run passes: https://github.com/temporalio/api-go/actions/runs/26121571272
It correctly synced temporalio/api#742 to api-go (on this branch): 2150dd5#diff-4253e2d69d45111060b6d9fb4cdc22f1ad5942e4836eabf57b6845b9e319e7c7

[semgrep-managed-scans]

Semgrep found 1 missing-explicit-permissions finding:

• .github/workflows/update-proto.yml
  • L22-76 - Triage

No explicit GITHUB_TOKEN permissions found at the workflow or job level. Add a permissions: block at the workflow root (applies to all jobs) or per job with least privilege (e.g., contents: read and only specific writes like pull-requests: write if needed).

[spkane31]

https://github.com/nexus-rpc/nexus-rpc-gen/blob/main/.github/workflows/ci.yml#L25-L36
Can we do the pnpm installation the same as nexus-rpc-gen

[long-nt-tran]

replying to capture offline discussion: keeping as is, since #261 changed the pnpm version in this repo to be a newer one. Would have to revert Makefile change if we installed it the same way as we do in nexus-rpc-gen