[ci] Fix macOS LUCI merge base (#6798)

Currently the repo tooling relies on `FETCH_HEAD` being set to
`origin/main` in CI, which is done in `prepare_tool.sh` for LUCI bots.
Longer term we should restructure the CI scripts to be explicit about
the base SHA instead of relying on `FETCH_HEAD`, but for now this adds
the missing `prepare_tool.sh` step to the new iOS LUCI tests so that
they compute diffs correctly.

Fixes flutter/flutter#116448

Author: stuartmorgan-g

.ci/targets/mac_ios_platform_tests.yaml

@@ -1,4 +1,6 @@
 tasks:
+  - name: prepare tool
+    script: .ci/scripts/prepare_tool.sh
   - name: create simulator
     script: .ci/scripts/create_simulator.sh
   - name: build examples