Support for multiple `masters`

[mschubert]

I've built my own overlay that I would like to pkgcheck using GHA. I'm depending on some ebuilds in science, so I've set the metadata/layout.conf to:

masters = gentoo science

However, when I'm running the pkgcheck action, it is syncing only the main tree and then reporting that it did not find the science repository (full log):

Sync gentoo repo
  /opt/hostedtoolcache/Python/3.11.4/x64/bin/pmaint sync gentoo
  *** syncing gentoo
  *** synced gentoo
Update repo metadata
  /opt/hostedtoolcache/Python/3.11.4/x64/bin/pmaint regen --dir /home/runner/.cache/pkgcheck/repos .
  pmaint regen: error: argument repo: repo init failed: repo 'mschubert' at path
      '/home/runner/work/overlay/overlay' has missing masters: 'science'

Is there a way to depend on additional overlays with this action?