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/outputs/index.ts

@@ -1,9 +1,7 @@
 import { setOutput } from '@actions/core'
 import { Inputs } from '../inputs'
-import { getBinDest } from '../utils'
 
-export function setOutputs(inputs: Inputs) {
-  const binDest = getBinDest(inputs)
+export function setOutputs(inputs: Inputs, binDest: string) {
   // NOTE: addPath is already called in installPnpm — do not call it again
   // here, as a second addPath would shadow the correct entry on Windows.
   setOutput('dest', inputs.dest)