Simplify gce inventory plugin injector

This consumes the change made in Ansible core
https://github.com/ansible/ansible/pull/54407
which is in Ansible 2.8, allowing the plugin
injection logic to share the script logic and
to be simplified

awx/main/tests/data/inventory/plugins/gce/env.json

@@ -1,3 +1,4 @@
 {
-    "ANSIBLE_TRANSFORM_INVALID_GROUP_CHARS": "never"
+    "ANSIBLE_TRANSFORM_INVALID_GROUP_CHARS": "never",
+    "GCE_CREDENTIALS_FILE_PATH": "{{ file_reference }}"
 }

awx/main/tests/data/inventory/plugins/gce/files/gcp_compute.yml

@@ -44,7 +44,6 @@ plugin: gcp_compute
 projects:
 - fooo
 retrieve_image_info: true
-service_account_file: {{ file_reference }}
 use_contrib_script_compatible_sanitization: true
 zones:
 - us-east4-a