CI: Fix false CI failures with package helper files

When touching helper files in category/package/files, they appear in git diff and are thus parsed by tools/repoman_changed_files.sh. This causes repoman to be run in category/package/files, which in turn causes a repoman error, since the files subdirectory is not a valid directory that contains ebuilds.

This commit fixes the CI pipeline by respecting only directores relative to the repository root with a single / in their path (i.e. the package diretories).

Signed-off-by: Adrian 'nex' Schollmeyer adrian.schollmeyer@fem.tu-ilmenau.de