[9.20] chg: ci: Replace the check_for_junit_xml anchor with a Python script

~~Where applicable, use the more detailed CMocka generated JUnit
reports which include subtest results and timings instead of the
one generated by Meson.~~

Prerequisites:
- bind9-qa!137

~~Closes #5511~~

Partial backport of MR !11100

Merge branch 'backport-5511-cmocka-junit-ouput-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11433