[macOS] Clean up gen_snapshot copy invocation (flutter#36461)

create_macos_gen_snapshots.py uses `xcrun bitcode_strip` to copy
gen_snapshot from its build directory to a suffixed gen_snapshot_arm64
or gen_snapshot_armv7, while stripping the bitcode segment, if any.

This simplifies the python invocation to use subprocess.check_call, and
also guarantees the process exits with failure in such a situation --
we weren't checking the return value of generate_gen_snapshot.

This is cleanup for issues encountered while working on merging
macOS/iOS gen_snapshot tooling.

Issue: flutter/flutter#103386
Issue: flutter/flutter#101138

sky/tools/create_macos_gen_snapshots.py

@@ -82,21 +82,9 @@ def generate_gen_snapshot(directory, destination):
     print('Cannot find gen_snapshot at %s' % gen_snapshot_dir)
     sys.exit(1)
 
-  command = [
+  subprocess.check_call([
       'xcrun', 'bitcode_strip', '-r', gen_snapshot_dir, '-o', destination
-  ]
-  process = subprocess.Popen(
-      command, stderr=subprocess.STDOUT, stdout=subprocess.PIPE
-  )
-  stdout, stderr = process.communicate()
-  exit_status = process.wait()
-
-  if exit_status != 0:
-    print(
-        'Error processing command with stdout[%s] and stderr[%s]' %
-        (stdout, stderr)
-    )
-    return 1
+  ])
 
 
 if __name__ == '__main__':