fix: add credentials_file_path input to root action.yaml (#246)

action.yml

@@ -59,6 +59,15 @@ inputs:
     default: false
     required: false
 
+  credentials_file_path:
+    description: |-
+      The path and name of the file to which to write the shared default
+      credentials. This option is only valid when
+      export_default_credentials=true. By default, the credentials will be
+      written to a new file in the root of GITHUB_WORKSPACE.
+    default: null
+    required: false
+
 runs:
   using: node12
   main: setup-gcloud/dist/index.js