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?

Please advise

[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).

[long-nt-tran]

closing this one, can't manually kick off a GHA from a fork. Will use a branch off upstream for this