Support setting shell via defaults.run (#343)

Note that we don't support the more general use of defaults, just its
run key for setting shell and working directory.

The documentation for defaults.run is here:
  https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#defaultsrun

Fixes #214.

Co-authored-by: Casey Lee <cplee@nektos.com>

pkg/runner/runner_test.go

@@ -55,6 +55,7 @@ func TestRunEvent(t *testing.T) {
 		{"commands", "push", ""},
 		{"workdir", "push", ""},
 		{"issue-228", "push", ""}, // TODO [igni]: Remove this once everything passes
+		{"defaults-run", "push", ""},
 	}
 	log.SetLevel(log.DebugLevel)