localtest: Fix regression and add some improvements

Commit a27543ba "gitlab-ci: Extend filtering by package to all retrieval jobs" broke localtest since PROJECTS_NAMESPACE was not set and no GitLab project could be properly retrieved.

The same commit made adding --filter-packages again redundant.

And then add auth support when accessing GitLab to make things work for downstreams.

added 4 commits

• 3300212d - 1 commit from branch master
• 550f8c00 - localtest: Fix regression after package filtering improvements
• 9f5b41e8 - localtest: Package filtering is now in the main pipeline
• c0fbce03 - localtest: Support instances requiring auth

Compare with previous version

added 6 commits

• c0fbce03...4434d931 - 3 commits from branch master
• 773bc4dd - localtest: Fix regression after package filtering improvements
• ad89a878 - localtest: Package filtering is now in the main pipeline
• 9a0a15d0 - localtest: Support instances requiring auth

Compare with previous version

merged