Use separate variable for flutter/flutter release hash

The previous change relies on the order of release of flutter/flutter then flutter/engine to use the correct release hash, which is unnecessary and less readable than using a dedicated variable for flutter/flutter that isn't overwritten.

Change-Id: I16c1deaf065212926966522cbdf2ccf3bda70284
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/41941
Reviewed-by: Casey Hillers <chillers@google.com>
Commit-Queue: Jesse Seales <jseales@google.com>
diff --git a/recipes/release/release_publish.py b/recipes/release/release_publish.py
index e5852cf..94cf63e 100644
--- a/recipes/release/release_publish.py
+++ b/recipes/release/release_publish.py
@@ -62,7 +62,7 @@
   assert api.platform.is_linux or api.platform.is_mac
 
   with api.step.nest('checkout flutter release branch'):
-    rel_hash = api.repo_util.checkout(
+    flutter_rel_hash = api.repo_util.checkout(
       'flutter',
       flutter_checkout,
       url=flutter_git_url,
@@ -100,7 +100,7 @@
     env = env_flutter if repo=='flutter' else env_engine
     env_prefixes = env_flutter_prefixes if repo=='flutter' else env_engine_prefixes
     checkout = flutter_checkout if repo=='flutter' else engine_checkout
-    rel_hash = rel_hash if repo=='flutter' else GetEngineVersion(api, flutter_checkout)
+    rel_hash = flutter_rel_hash if repo=='flutter' else GetEngineVersion(api, flutter_checkout)
     with api.context(env=env, env_prefixes=env_prefixes, cwd=checkout):
       token_decrypted = api.path['cleanup'].join('token.txt')
       api.kms.get_secret('flutter-release-github-token.encrypted', token_decrypted)