Add transition aware reverse dependency builds

Add a new SALSA_CI_BUILD_REVERSE_DEPENDENCIES_TRANSITION_AFFECTED_REGEX variable for build reverse dependency jobs.

When the variable is unset, the job keeps the existing ratt -direct-rdeps behavior. When it is set, the job passes the regex to ratt -transition_affected instead.

This is useful for library transitions, where the relevant rebuild set is based on packages depending on the affected runtime library names, rather than all reverse build dependencies of the binaries in the .changes file.

The README now documents how to use the new variable.

Closes: #571 (closed)