Updates for new spack-config

[CodeGat]

References ACCESS-NRI/spack-config#41
Closes #349

Background

There is an update to spack-config (see ACCESS-NRI/spack-config#110), which makes use of the includes.yaml, and a move away from the spack-enable.bash. This PR updates the workflow to use setup-env.sh, and adds a new script to set up service users.

Furthermore, we revert to essentially pre-v8 behavior in which instead of builtin-spack-packages being a lever to pull in config/settings.json, it is informed solely by spack-config.

The PR

• Swap from spack-enable.bash to setup-env.sh
• Add tools/service-user for setup of service users, update README
• Remove updating of builtin via settings.json, now informed by spack-config

Testing

Tested in Prerelease via ACCESS-NRI/ACCESS-TEST#65

[CodeGat]

@CodeGat don't forget to update the hash of spack in config/settings.json post-merge of ACCESS-NRI/spack-config#110 !

EDIT: Done in f0e4d61

[harshula]

Reviewed via Zoom VC