Merge gitea/act into act/

Merges the `gitea.com/gitea/act` fork into this repository as the `act/`
directory and consumes it as a local package. The `replace github.com/nektos/act
=> gitea.com/gitea/act` directive is removed; act's dependencies are merged
into the root `go.mod`.

- Imports rewritten: `github.com/nektos/act/pkg/...` → `gitea.com/gitea/act_runner/act/...`
  (flattened — `pkg/` boundary dropped to match the layout forgejo-runner adopted).
- Dropped act's CLI (`cmd/`, `main.go`) and all upstream project files; kept
  the library tree + `LICENSE`.
- Added `// Copyright <year> The Gitea Authors ...` / `// Copyright <year> nektos`
  headers to 104 `.go` files.
- Pre-existing act lint violations annotated inline with
  `//nolint:<linter> // pre-existing issue from nektos/act`.
  `.golangci.yml` is unchanged vs `main`.
- Makefile test target: `-race -short` (matches forgejo-runner).
- Pre-existing integration test failures fixed: race in parallel executor
  (atomic counters); TestSetupEnv / command_test / expression_test /
  run_context_test updated to match gitea fork runtime; TestJobExecutor and
  TestActionCache gated on `testing.Short()`.

Full `gitea/act` commit history is reachable via the second parent.

Co-Authored-By: Claude (Opus 4.7) <noreply@anthropic.com>

act/runner/testdata/environment-variables/push.yml

@@ -0,0 +1,33 @@
+name: environment variables
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Test on job level
+        run: |
+          echo \$UPPER=$UPPER
+          echo \$upper=$upper
+          echo \$LOWER=$LOWER
+          echo \$lower=$lower
+          [[ "$UPPER" = "UPPER" ]] || exit 1
+          [[ "$upper" = "" ]] || exit 1
+          [[ "$LOWER" = "" ]] || exit 1
+          [[ "$lower" = "lower" ]] || exit 1
+      - name: Test on step level
+        run: |
+          echo \$UPPER=$UPPER
+          echo \$upper=$upper
+          echo \$LOWER=$LOWER
+          echo \$lower=$lower
+          [[ "$UPPER" = "upper" ]] || exit 1
+          [[ "$upper" = "" ]] || exit 1
+          [[ "$LOWER" = "" ]] || exit 1
+          [[ "$lower" = "LOWER" ]] || exit 1
+        env:
+          UPPER: upper
+          lower: LOWER
+    env:
+      UPPER: UPPER
+      lower: lower