Change engine merge method back to squash (#24111)

We need to wait the fix in
https://skia-review.googlesource.com/c/buildbot/+/169724
to land in order to enable the rebase merge method again.

TBR: goderbauer

We'll land this quickly without waiting for presubmit checks
to unblock our engine roller.

bin/internal/engine.merge_method

-rebase
+squash