fix ${cvmfs_repo_name} in EESSI_WRITABLE_OVERLAY when merging all lower directories

[TopRichard]

When merging all lower directories, we are ending up in:

singularity  run  --fusemount container:cvmfs2 pilot.nessi.no /cvmfs_ro/pilot.nessi.no --fusemount container:fuse-overlayfs -o lowerdir=/tmp/nessibot/14903/NESSI/lower_dirs:/cvmfs_ro/ -o upperdir=/tmp/pilot.nessi.no/overlay-upper -o workdir=/tmp/pilot.nessi.no/overlay-work /cvmfs/pilot.nessi.no /tmp/nessibot/14903/NESSI/eessi.1kJcn9xcrp/ghcr.io_eessi_build_node_debian11.sif ./EESSI-remove-software.sh

[nessi-bot]

Instance AWS-MC-NESSI is configured to build for:

• architectures: x86_64/generic, x86_64/intel/skylake_avx512, x86_64/amd/zen2, aarch64/generic
• repositories: nessi-2023.06-swl-deb11, nessi-2023.06-swl-deb10, nessi-2023.06-cl

[nessi-bot]

Instance Fram-NESSI is configured to build for:

• architectures: x86_64/generic, x86_64/intel/broadwell
• repositories: nessi-2023.06-swl-deb11, nessi-2023.06-cl, nessi-2023.06-swl-deb10

[nessi-bot]

Instance Saga-NESSI is configured to build for:

• architectures: x86_64/intel/skylake_avx512, x86_64/intel/broadwell, x86_64/generic
• repositories: nessi-2023.06-cl, nessi-2023.06-swl-deb11, nessi-2023.06-swl-deb10

[TopRichard]

Code change, no build is required, will be tested in #428

[justadejavu]

Test required