Remove /cvmfs/ in path to overlay upper directory in tarball script

[bedroge]

Follow-up to #635 , which didn't fully fix the issue. The cvmfs_repo variable not only contained the repo name, but also had a leading /cvmfs. This has to be removed when setting the path to the overlay upper directory, which looks like /tmp/<repo name>/overlay-upper.

[eessi-bot]

Instance eessi-bot-mc-aws is configured to build for:

• architectures: x86_64/generic, x86_64/intel/haswell, x86_64/intel/skylake_avx512, x86_64/amd/zen2, x86_64/amd/zen3, aarch64/generic, aarch64/neoverse_n1, aarch64/neoverse_v1
• repositories: eessi.io-2023.06-compat, eessi-hpc.org-2023.06-software, eessi-hpc.org-2023.06-compat, eessi.io-2023.06-software

[eessi-bot]

Instance eessi-bot-mc-azure is configured to build for:

• architectures: x86_64/amd/zen4
• repositories: eessi-hpc.org-2023.06-compat, eessi-hpc.org-2023.06-software, eessi.io-2023.06-software, eessi.io-2023.06-compat

[bedroge]

Let's test this in #634 before merging it.

[bedroge]

The build completed successfully (#634), so this fix indeed solves the issue.