Pin Bazel to 4.2.2 (#194)

Since OSS-Fuzz uses Bazelisk to invoke Bazel and rules_fuzzing does not
pin a Bazel version, the latest stable release of Bazel will be used.
This can (and did, twice now) unexpectedly break the build.

We should intead pin a Bazel version and consciously update it when we
add support for a new version.
diff --git a/.bazelversion b/.bazelversion
new file mode 100644
index 0000000..af8c8ec
--- /dev/null
+++ b/.bazelversion
@@ -0,0 +1 @@