Add support for changing the GITHUB_ACTOR (#229)

This adds the `-a` flag when running `act` to change the username of the GITHUB_ACTOR environment variable

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

cmd/input.go

@@ -7,6 +7,7 @@ import (
 
 // Input contains the input for the root command
 type Input struct {
+	actor           string
 	workdir         string
 	workflowsPath   string
 	eventPath       string