ci: presubmit-windows segmentation fault during pnpm install with .pnpmfile.cjs

[quirogas]

During the presubmit-windows GitHub Actions workflow, unit tests fail on Windows runners with a segmentation fault during pnpm install:

```text
/d/a/google-cloud-node/google-cloud-node/ci/run_single_test.sh: line 44:  2125 Segmentation fault  pnpm install --ignore-scripts --engine-strict --prod --pnpmfile "${PROJECT_ROOT}/.pnpmfile.cjs"

Root Cause

ci/run_single_test.sh computes PROJECT_ROOT using realpath, which returns a POSIX path in Git Bash (e.g/d/a/google-cloud-node/...). When --pnpmfile is passed with a Unix-style POSIX path to native Windows Node.js/PNPM processes, the module loader fails to resolve the drive path and crashes with a SIGSEGV (exit status 2125).