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

act/runner/expression_test.go

@@ -185,7 +185,6 @@ func TestInterpolate(t *testing.T) {
 func updateTestExpressionWorkflow(t *testing.T, tables []struct {
 	in  string
 	out string
-	//wantErr bool
 }, rc *RunContext) {
 
 	var envs string