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.

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

[ocaisa]

This creates an alternative installation setup that is dependent on whether the ${eessi_extend_module} is available or not. Rather than do that we should just have a script that ensures this module is in place (something we already have for EasyBuild itself, https://github.com/EESSI/software-layer/blob/2023.06-software.eessi.io/load_easybuild_module.sh).

I'll look into extending the existing script to also/alternatively install EESSI-extend

[ocaisa]

Suggestion for this in trz42#76