Add go-version-file option (#62)

action.yml

@@ -4,6 +4,8 @@ author: 'GitHub'
 inputs:
   go-version:
     description: 'The Go version to download (if necessary) and use. Supports semver spec and ranges.'
+  go-version-file:
+    description: 'Path to the go.mod file.'
   check-latest:
     description: 'Set this option to true if you want the action to always check for the latest available version that satisfies the version spec'
     default: false