Only build the x64 variant of Fuchsia on the try-jobs. (#12206)

Halves the presubmit times.

.cirrus.yml

@@ -78,7 +78,7 @@ task:
     - name: build_fuchsia_artifacts
       compile_fuchsia_script: |
         cd $ENGINE_PATH/src
-        ./flutter/tools/fuchsia/build_fuchsia_artifacts.py --engine-version HEAD --runtime-mode debug --no-lto
+        ./flutter/tools/fuchsia/build_fuchsia_artifacts.py --engine-version HEAD --runtime-mode debug --no-lto --archs x64
         cd $ENGINE_PATH/src/flutter
         ./ci/build_flutter_runner_tests.sh
 

tools/fuchsia/build_fuchsia_artifacts.py

@@ -233,6 +233,12 @@ def main():
       choices=['debug', 'profile', 'release', 'all'],
       default='all')
 
+  parser.add_argument(
+      '--archs',
+      type=str,
+      choices=['x64', 'arm64', 'all'],
+      default='all')
+
   parser.add_argument(
       '--no-lto',
       action='store_true',
@@ -243,7 +249,7 @@ def main():
   RemoveDirectoryIfExists(_bucket_directory)
   build_mode = args.runtime_mode
 
-  archs = ['x64', 'arm64']
+  archs = ['x64', 'arm64'] if args.archs == 'all' else [args.archs]
   runtime_modes = ['debug', 'profile', 'release']
   product_modes = [False, False, True]