ci: don't hide tc-admin stderr (#233)

tc-admin diff uses the same exit code (1) both for "there was an error"
and "I found differences", so its error output is basically the only way
we can differentiate currently.

taskcluster/fxci_config_taskgraph/optimizations.py

@@ -28,7 +28,7 @@ def _get_modified_worker_pools(self) -> set[str]:
         ]
         env = os.environ.copy()
         env["TASKCLUSTER_ROOT_URL"] = FIREFOXCI_ROOT_URL
-        proc = subprocess.run(cmd, capture_output=True, text=True, env=env)
+        proc = subprocess.run(cmd, stdout=subprocess.PIPE, text=True, env=env)
         lines = [line for line in proc.stdout.splitlines() if line.startswith("!")]
 
         worker_pools = set()