ci: give the release dry-run an unused android version code

Play rejects a version code it has already seen, and between releases
pubspec's code is by definition already published — so every scheduled
dry-run died on "Version code 69 has already been used" before reaching
any of the credential, signing, or store validation it exists to check.
It has been failing on that since 5.1.0 shipped.

Overrides the build number with epoch seconds, which is always unused and
always increasing. The bundle is validate_only, never published, so the
value is discarded. Only downside is the dry-run can no longer catch a
forgotten version bump, which the real release catches anyway.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/release-dryrun.yml

@@ -69,8 +69,16 @@ jobs:
           echo storeFile=$GITHUB_WORKSPACE/android/upload-keystore.jks >> $KEY_FILE_PATH
 
       # Build the Android app bundle (AAB) and pass the environment variables.
+      #
+      # The version code is overridden because Play rejects one it has already
+      # seen, and pubspec's code is by definition already published between
+      # releases — so the dry-run would fail on every scheduled run and never
+      # reach the validation it exists to perform. Epoch seconds is always
+      # unused, always increasing, and well under Play's 2100000000 cap. The
+      # bundle is only ever validated, never published, so the value is thrown
+      # away. versionName still comes from pubspec.
       - name: Build the Android AAB
-        run: flutter build appbundle --dart-define CLIENT_ID=$CLIENT_ID --dart-define SECRET=$SECRET
+        run: flutter build appbundle --build-number=$(date +%s) --dart-define CLIENT_ID=$CLIENT_ID --dart-define SECRET=$SECRET
 
       # Validate the AAB and credentials against Google Play without publishing.
       - name: Validate release with fastlane (no publish)