Substitute slashes for hyphens in BASE_REF and HEAD_REF consistently.
Substitute slashes for hyphens in BASE_REF and HEAD_REF consistently.

Dependabot creates a branch with a slash in the name, but then it is not read
by the Makefile rule at the top of the file because it already exists in the
environment, so the `tr` substitution didn't kick in.

google/mtail,SuperQ/mtail,SuperQ/mtail,google/mtail