add missing /cvmfs/ in $EESSI_CVMFS_REPO_OVERRIDE defined by bot/build.sh and bot/test.sh

[boegel]

This is required after the changes in #618, which used to include this in eessi_container.sh (but not anymore):

export EESSI_CVMFS_REPO_OVERRIDE="/cvmfs/${repo_name}"

Removing that exposed the incorrect setting of $EESSI_CVMFS_REPO_OVERRIDE in bot/build.sh, which never was correct without the /cvmfs/

[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

[eessi-build-deploy-bot-deucalion]

Instance boegel-bot-deucalion is configured to build for:

• architectures: aarch64/a64fx
• repositories: eessi.io-2023.06-software

[boegel]

Build for #632 is actually running now, which confirms this fix