fix: composite action input pollution (#2348)

* fix: composite action input pollution

* fix run steps

* fix missing defaults in post after env cleanup

* fix test to make more sense

* Add tests and simplify change

---------

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pkg/runner/testdata/uses-composite-check-for-input-shadowing/push.yml

@@ -0,0 +1,12 @@
+name: uses-composite-with-pre-and-post-steps
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: echo -n "STEP_OUTPUT_TEST=empty" >> $GITHUB_ENV
+      - uses: ./uses-composite-check-for-input-shadowing/composite_action
+        # with:
+          # cache: other