use develop branch for PRs that target removed 5.0.x branch in fetch_files_from_pr

[boegel]

The long-lived 5.0.x branch that was used for preparing EasyBuild v5.0.0 was collapsed into the develop branch and removed shortly after the release of EasyBuild v5.0.0.

That causes trouble when --from-pr or --include-easyblocks-from-pr is used on PRs that were targeting the 5.0.x branch, for example:

$ eb --from-pr 22227 -Dr
== Temporary log file in case of crash /tmp/eb-w6t5qm45/easybuild-a07fthw5.log
pr_target_branch='5.0.x'
ERROR: No branch with name 5.0.x found in repo easybuilders/easybuild-easyconfigs: auto_merge_prs, develop, dual_serial_mpi, main, ocaisa-patch-1

Since 5.0.x was collapsed into develop, we can easily fix this by using develop instead

[lexming]

LGTM