fix: drop patchPnpmEnv so standalone+self-update works on Windows

`patchPnpmEnv` prepended `dest/node_modules/.bin` to PATH before
spawning `pnpm install` / `pnpm store prune`. On Windows in standalone
mode, `.bin/pnpm.cmd` is an npm-created shim that always points at the
BOOTSTRAP pnpm (currently 11.0.4) — the binary npm linked when it
installed `@pnpm/exe` into `node_modules`. The self-updated pnpm
written by `pnpm self-update` lives at `$PNPM_HOME/bin`, which is
separately added to PATH via `addPath()` in install-pnpm.

When the user requested a pnpm version different from the bootstrap
under `standalone: true` on Windows, patchPnpmEnv's `.bin` entry
shadowed the self-updated `$PNPM_HOME/bin` and the action's internal
`pnpm install` ran on the bootstrap. On a pnpm 11.0.x bootstrap this
broke any 11.1+ install flag (e.g. `--no-runtime`), reporting:

    ERROR  Unknown option: 'runtime'

POSIX standalone got lucky because `.bin` and `$PNPM_HOME` resolve to
the same directory there. Non-standalone never tripped on this since
the `.bin/pnpm` symlink for a regular `pnpm` package keeps working
across self-updates.

Removed `patchPnpmEnv` and the now-empty `src/utils/` module.
`spawnSync` now inherits `process.env`, whose PATH is already
correctly fronted by `$PNPM_HOME/bin` and `$PNPM_HOME` via the
`addPath` calls in install-pnpm.

Added `standalone_windows_self_update` to test.yaml as a regression
guard: standalone on Windows + target 11.1.0 + `run_install` with
`--no-runtime`. With the previous code, the install would have run
under the bootstrap (11.0.4) and errored on the unknown flag.

Originally found while building pnpm/setup (the new combined
pnpm + runtime action).

.github/workflows/test.yaml

@@ -194,6 +194,54 @@ jobs:
           pnpm add is-odd
         shell: bash
 
+  standalone_windows_self_update:
+    # Regression guard for the patchPnpmEnv PATH-shadow bug. When
+    # standalone: true on Windows AND the requested pnpm differs from the
+    # bootstrap, the previous patchPnpmEnv prepended node_modules/.bin to
+    # PATH; that directory contains an npm-created pnpm.cmd shim pointing
+    # at the BOOTSTRAP pnpm, which shadowed the self-updated pnpm at
+    # $PNPM_HOME/bin and caused `pnpm install` inside the action to run
+    # under the bootstrap version. Exercising a newer-pnpm-only flag
+    # (`--no-runtime`, added in 11.1.0) makes the regression assertable:
+    # if the bootstrap (11.0.4) handles the install, it errors with
+    # "Unknown option: 'runtime'".
+    name: 'Standalone Windows self-update (PATH regression)'
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+
+      - name: Set up package.json with a minimal manifest
+        # run_install needs a manifest to install against. Removing the
+        # repo's existing pnpm-lock.yaml avoids frozen-lockfile mismatch.
+        run: |
+          rm -f pnpm-lock.yaml
+          echo '{"name":"sw","private":true,"packageManager":"pnpm@11.1.0"}' > package.json
+        shell: bash
+
+      - name: Run the action
+        uses: ./
+        with:
+          version: 11.1.0
+          standalone: true
+          run_install: |
+            args: ['--no-runtime']
+
+      - name: 'Test: pnpm install completed under the self-updated pnpm'
+        # If the bug recurs, the previous step's run_install will have failed
+        # the job with "Unknown option: 'runtime'", so reaching this step
+        # implies success. Still verify the version on PATH matches request.
+        env:
+          REQUIRED: '11.1.0'
+        run: |
+          set -e
+          actual="$(pnpm --version)"
+          echo "pnpm --version: ${actual}"
+          if [ "${actual}" != "${REQUIRED}" ]; then
+            echo "Expected pnpm ${REQUIRED}, got ${actual}"
+            exit 1
+          fi
+        shell: bash
+
   cache_store_path:
     # Regression guard for #233. When package.json pins a pnpm major that
     # differs from the bootstrap pnpm's major, the bootstrap reports its