add easystacks arg and bootstrap improvements

[trz42]

Adds command line arg --easystacks to install script EESSI-install-software.sh. This can be used for interactively building software using one or multiple easystack files (comma-separated). Starting builds for a new architecture could be as simple as

export EESSI_SOFTWARE_SUBDIR_OVERRIDE=aarch64/neoverse_v2
./run_in_compat_layer_env.sh \
  EESSI-install-software.sh \
    --easystacks ${HOME}/es/eessi-2023.06-eb-4.9.4-001-system.yml,${HOME}/es/eessi-2023.06-eb-4.9.4-2023b.yml

provided /cvmfs/software.eessi.io is writable as when running eessi_container.sh --access rw.

The below is not needed after pulling in previously merged requests.
Also improves bootstrapping of a new architecture:

• adds software directory to ${EESSI_PREFIX}/software/${EESSI_OS_TYPE}/${EESSI_SOFTWARE_SUBDIR_OVERRIDE} as its existence is tested by the EESSI/2023.06 module
• checks if the module EESSI-extend/${EESSI_VERSION}-easybuild exists
  • if so, it loads it
  • if not, it doesn't load it but sets both ${EASYBUILD_INSTALLPATH} and ${EASYBUILD_EXPERIMENTAL} which are needed to install EasyBuild/SOME_VERSION and EESSI-extend/${EESSI_VERSION}-easybuild

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

[boegel]

@trz42 Can you sync this so we can get it merged?

[trz42]

@trz42 Can you sync this so we can get it merged?

Updated the PR. It now only includes the changes related to passing an easystack file as argument to the script, then only that is processed instead of easystack files being determined from a .diff file.