Change possible password fields

GOOGLE_APPLICATION_CREDENTIALS "should point to a file that defines the credentials", and should not be the contents of the credentials to follow Google's convention.

See https://developers.google.com/identity/protocols/application-default-credentials#howtheywork.

cmd/drone-docker-gcr/main.go

@@ -19,7 +19,7 @@ func main() {
 			"PLUGIN_JSON_KEY",
 			"GCR_JSON_KEY",
 			"GOOGLE_CREDENTIALS",
-			"GOOGLE_APPLICATION_CREDENTIALS",
+			"TOKEN",
 		)
 	)