Always re-run release builders from ToT. (#4687)

Closes flutter/flutter#168944.

Author: matanlurey

app_dart/lib/src/service/luci_build_service.dart

@@ -1153,7 +1153,8 @@ class LuciBuildService {
     final result = await _buildBucketClient.scheduleBuild(
       bbv2.ScheduleBuildRequest(
         builder: builderId,
-        exe: bbv2.Executable(cipdVersion: 'refs/heads/${commit.branch}'),
+        // Release builds intentionally use ToT for recipe branches.
+        exe: null,
         gitilesCommit: bbv2.GitilesCommit(
           project: 'mirrors/${commit.slug.name}',
           host: 'flutter.googlesource.com',

app_dart/test/service/luci_build_service/rerun_dart_internal_test.dart

@@ -141,8 +141,9 @@ void main() {
             ),
       );
       expect(
-        request.exe,
-        bbv2.Executable(cipdVersion: 'refs/heads/flutter-0.42-candidate.0'),
+        request.hasExe(),
+        isFalse,
+        reason: 'Release recipes always run from ToT',
       );
       expect(
         request.gitilesCommit,
@@ -252,8 +253,9 @@ void main() {
             ),
       );
       expect(
-        request.exe,
-        bbv2.Executable(cipdVersion: 'refs/heads/flutter-0.42-candidate.0'),
+        request.hasExe(),
+        isFalse,
+        reason: 'Release recipes always run from ToT',
       );
       expect(
         request.gitilesCommit,
@@ -335,8 +337,9 @@ void main() {
             ),
       );
       expect(
-        request.exe,
-        bbv2.Executable(cipdVersion: 'refs/heads/flutter-0.42-candidate.0'),
+        request.hasExe(),
+        isFalse,
+        reason: 'Release recipes always run from ToT',
       );
       expect(
         request.gitilesCommit,