fix: ensure the xcodeproj project generation uses the correct Bazel binary in the firebase_example

[cgrindel]

The rules_xcodeproj project generator calls Bazel. By default, it looks for bazel on the PATH. When running under rules_bazel_integration_test this will pick up the default Bazel version on the system. If that is Bazelisk, it will pull down the latest Bazel. The fix is to explicitly tell xcodeproj which Bazel binary to use.

• Add tools/bazel to the firebase_example. It inspects environment variables to determine the Bazel version to use and writes tools/bazel_for_xcodeproj which executes Bazel commands against the correct Bazel binary.
• Use sanbox for rules_xcodeproj builds in firebase example. This avoids the duplicate definition error.
• Add print_and_run bash function to examples/firebase_example/do_test to ease debugging future issues.