fix: bin_dest output points to self-updated pnpm, not bootstrap (#247)

`pnpm self-update <version>` writes the target binary to
`${PNPM_HOME}/bin/`, leaving the bootstrap symlink at `${PNPM_HOME}/pnpm`
untouched. The `bin_dest` output was set to `${PNPM_HOME}`, so consumers
invoking `${{ steps.pnpm.outputs.bin_dest }}/pnpm` got the bootstrap
version (currently 11.0.4) instead of the version they requested.

PATH lookup hid the bug: `${PNPM_HOME}/bin` was prepended ahead of
`${PNPM_HOME}`, so `pnpm` resolved from PATH was the right one. Existing
version-respect tests only checked `pnpm --version`, not `bin_dest`.

Resolve `binDest` inside `runSelfInstaller` (target lives in
`${PNPM_HOME}/bin` after self-update, otherwise stays at `${PNPM_HOME}`)
and plumb it through to `setOutputs`. Add a regression test that invokes
`${bin_dest}/pnpm --version` directly across Linux/macOS/Windows.

src/index.ts

@@ -20,9 +20,10 @@ async function main() {
 async function runMain(inputs: Inputs) {
   saveState('is_post', 'true')
 
-  await installPnpm(inputs)
+  const binDest = await installPnpm(inputs)
+  if (binDest === undefined) return
   console.log('Installation Completed!')
-  setOutputs(inputs)
+  setOutputs(inputs, binDest)
 
   await restoreCache(inputs)