[9.20] chg: ci: Don't overwrite JUnit file generated by pytest in post-processing

When both input and output files are the same, it creates unnecesary
troubles in debugging issues with the JUnit files.

Keep the Pytest-generated file in the artifacts and output the checked
version as a new file.

Companion MR: https://gitlab.isc.org/isc-projects/bind9-qa/-/merge_requests/155

Backport of MR !11430

Merge branch 'backport-stepan/dont-overwrite-pytest-junit-xml-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11431