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

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).