Runtime version detection breaks when pnpm writes [WARN] to stdout #57

What happens

Every job using pnpm/setup@v2.1.0 with a runtime to install gets a warning annotation:

Unable to determine the installed runtime versions: Unexpected token 'W', "[WARN] Usin"... is not valid JSON

The runtime itself installs fine — only the version refinement (and the cache key it feeds) falls back.

Why

getInstalledRuntimeVersions (src/install-runtime/index.ts) does:

const stdout = await runPnpmForOutput(binDest, ['list', '--global', '--json', '--depth', '0'])
const listing = JSON.parse(stdout)

pnpm 12 writes a warning to stdout, ahead of the JSON, when the project configures pmOnFail:

[WARN] Using --global skips the package manager check for this project

so JSON.parse sees [WARN] Usin....

Repro (pnpm 12.4.1)

mkdir repro && cd repro
printf '{"name":"t","version":"1.0.0"}' > package.json
printf 'pmOnFail: warn\n' > pnpm-workspace.yaml   # `error` does it too; removing the key does not
pnpm list --global --json --depth 0 | head -1
# [WARN] Using --global skips the package manager check for this project

Suggested fix

pnpm list --reporter=silent --global --json --depth 0 returns clean JSON in the same repro. Alternatively, slice from the first [ before parsing, so any future stdout noise is tolerated — the comment above the function already says this path must never break setup.

Happy to send a PR if you'd like.