Fetch the unit eval commit before checking it out (#42636)

Release Notes:

- N/A

script/run-unit-evals

@@ -3,7 +3,7 @@
 set -euxo pipefail
 
 if [ -n "${UNIT_EVAL_COMMIT:-}" ]; then
-  git checkout "$UNIT_EVAL_COMMIT"
+  git fetch origin "$UNIT_EVAL_COMMIT" && git checkout "$UNIT_EVAL_COMMIT"
 fi
 
 GPUI_TEST_TIMEOUT=1500 cargo nextest run --workspace --no-fail-fast --features unit-eval --no-capture -E 'test(::eval_)'