stable containers use potentially outdated epic@main

The `@main` version is the default version defined here
https://github.com/eic/containers/blob/892e0b72770307439a4dc654709fd1fa5b8e5c27/spack-environment/xl/spack.yaml#L34
that is replaced in
https://github.com/eic/containers/blob/892e0b72770307439a4dc654709fd1fa5b8e5c27/containers/eic/Dockerfile#L187-L192
that is not overridden in default/stable arguments:
https://github.com/eic/containers/blob/892e0b72770307439a4dc654709fd1fa5b8e5c27/.gitlab-ci.yml#L441-L446

This is resulting in:
```
singularity exec /cvmfs/singularity.opensciencegrid.org/eicweb/eic_xl:nightly spack find epic
epic@25.06.0 epic@25.06.1 epic@25.07.0 epic@25.08.0 epic@25.09.0 epic@git.dfc8b03a39d293e463be8e159cddeee0a8732356=main
```
but
```
singularity exec /cvmfs/singularity.opensciencegrid.org/eicweb/eic_xl:25.09.0-stable spack find epic
epic@25.06.0 epic@25.06.1 epic@25.07.0 epic@25.08.0 epic@25.09.0 epic@main
```

The reason why `@main` is a worser version spec compared to `@git.${EPIC_VERSION}=main` is that the prior may be using a cached revision, which may be out of date. A particular consequence of this is that release containers can have
/opt/detectors/epic-main that is older than the latest /opt/detectors/epic-YY.MM.X .

The `epic` geometry package is a particular special case because other packages like EICrecon have a version pointing to a tagged release (updated manually on each container release).