add support for alternative artefacts checks in check-build.sh

[smoors]

if ALT_ARTEFACTS=1 is set, checking the tarball is skipped. instead, check-build-artefacts.sh is sourced, which may do alternative artefacts checks.

by default, ALT_ARTEFACTS=0, thus no change in behavior should be observed

this is useful for installations that are done directly on the system instead of CVMFS.

the changes are rather small if you hide the whitespace differences ;)

@boegel @laraPPr

[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.io-2023.06-compat, eessi.io-2023.06-software