Make project_id an optional input (#383)

The code and readme specify the `project_id` as optional, but it is
marked as required in the `action.yml` file. Marking it as optional

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Signed-off-by: Ayush Goyal <[email protected]>

Author: DefCon-007

action.yml

@@ -8,7 +8,7 @@ inputs:
     description: |-
       The Google Cloud Project ID. If unspecified, it is inherited from the
       environment.
-    required: true
+    required: false
 
   working_directory:
     description: |-