`diff_test::failure_message` can be used to execute sub-shells

Adding back-ticks to fail messages creates a sub-shell that leads to odd and/or undesirable behavior
https://github.com/bazelbuild/rules_python/commit/7740b22d0bae942af0797967f2617daa19834cb3#r64135831

The use of [diff_test::failure_message](https://github.com/bazelbuild/bazel-skylib/blob/1.1.1/rules/diff_test.bzl#L127) should not cause any additional commands to be executed.

Start with rules/diff_test.bzl at the diff_test::failure_message implementation linked in the issue. Check how the failure message is passed to Bazel and verify that back-tick content cannot trigger additional command execution; done means the failure message is displayed without running a sub-shell.

Written by the indexing model from the issue text.