GitHub env file support (#426)

* Upgrade to the official golangci-lint action and fix some issues it found

* Update deps

* Remove a shadow warning

* Initialize the splitPattern only once

* Initial attempt at supporting $GITHUB_ENV

Needs some polishing and tests

* Now it's actually working

* Replace golang.org/x/crypto/ssh/terminal with golang.org/x/term

* Disable the issue-228 test again

* The linter is picky

* Discovered that the workflow/envs.txt had to exist in certain cases

* Fix small linter issue

pkg/runner/runner_test.go

@@ -92,7 +92,7 @@ func TestRunEvent(t *testing.T) {
 		{"testdata", "matrix-include-exclude", "push", "", platforms},
 		{"testdata", "commands", "push", "", platforms},
 		{"testdata", "workdir", "push", "", platforms},
-		//{"testdata", "issue-228", "push", "", platforms}, // TODO [igni]: Remove this once everything passes
+		// {"testdata", "issue-228", "push", "", platforms}, // TODO [igni]: Remove this once everything passes
 		{"testdata", "defaults-run", "push", "", platforms},
 	}
 	log.SetLevel(log.DebugLevel)