Default to the latest Xcode version if no version is specified or requested version is not available. (#1900)

I think we should prefer the latest Xcode version and instead of the
oldest one on Bazel CI.

The latest version contains many bug fixes that would address
`//src/test/shell/bazel:bazel_determinism_test` in
https://buildkite.com/bazel/bazel-bazel/builds/27099#018e12ba-538c-4fa8-9e1d-d2a1ffe41d21
1 file changed