Pin Bazel version (#322)

Bazelisk currently automatically picks the latest Bazel release in CI.
This broke since Bazel 5 has been released a few weeks ago (the main
culprit being an out-of-date grpc-java). While ultimately we'd want to
fix the issue (which #319 seems to do), fixing Bazel's version moving
forward is likely a good thing to prevent this kind of unrelated
breakages and ensuring reproducible builds.

.bazelversion

@@ -0,0 +1 @@
+4.2.2