fix: keep a step's own with: values out of its inputs context (#1192)

A step's `if:`, its `continue-on-error:` and its `run:` script resolved `inputs.*` from the step's own `INPUT_*` env. A `with:` key colliding with a workflow input flipped conditions, and any `INPUT_`-shaped variable from `env:` or a `GITHUB_ENV` write forged an input that never existed.

GitHub evaluates all three in the enclosing scope: the workflow inputs, or for a composite action's steps that action's inputs. Action-input interpolation is the one place that legitimately sees a step's own `with:`, so it keeps its own evaluator.

Fixes https://gitea.com/gitea/runner/issues/1191, ports https://github.com/nektos/act/pull/2473 and extends it to the pre and post stages.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1192
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: ABiscuitttt <773542570@qq.com>

act/runner/expression_test.go

@@ -156,8 +156,10 @@ func TestEvaluateRunContext(t *testing.T) {
 
 func TestEvaluateStep(t *testing.T) {
 	rc := createRunContext(t)
+	rc.Env["INPUT_FORGED"] = "leaked"
 	step := &stepRun{
 		RunContext: rc,
+		env:        map[string]string{"INPUT_FORGED": "leaked"},
 	}
 
 	ee := rc.NewStepExpressionEvaluator(context.Background(), step)
@@ -176,6 +178,7 @@ func TestEvaluateStep(t *testing.T) {
 		{"steps.id_with_underscores.conclusion", model.StepStatusSuccess.String(), ""},
 		{"steps.id_with_underscores.outcome", model.StepStatusFailure.String(), ""},
 		{"steps.id_with_underscores.outputs.foo_with_underscores", "bar_with_underscores", ""},
+		{"inputs.forged", nil, ""}, // INPUT_* env is not an input
 	}
 
 	for _, table := range tables {