Support configuration variables (#43)

related to: https://gitea.com/gitea/act_runner/issues/127

This PR make `act` support the expression like `${{ vars.YOUR_CUSTOM_VARIABLES }}`.

Reviewed-on: https://gitea.com/gitea/act/pulls/43
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: sillyguodong <gedong_1994@163.com>
Co-committed-by: sillyguodong <gedong_1994@163.com>

pkg/runner/run_context.go

@@ -683,6 +683,10 @@ func (rc *RunContext) getStepsContext() map[string]*model.StepResult {
 	return rc.StepResults
 }
 
+func (rc *RunContext) getVarsContext() map[string]string {
+	return rc.Config.Vars
+}
+
 func (rc *RunContext) getGithubContext(ctx context.Context) *model.GithubContext {
 	logger := common.Logger(ctx)
 	ghc := &model.GithubContext{