Sherpa recipe notes/likely issues

[jeanconn]

Javier's notes from a skare3_tools issue:

I just took a few minutes to look into the sherpa build again. I'm not very inclined to fix it, but indeed I must have built it using a SHERPA_TAG environment variable defined.

The sherpa recipe has at least a few other issues (other than the use of the environment):

• ska_builder.py can't be used with this recipe (as noted in its add build recipe for sherpa #1349) and the reason is that it fails the same way as skare3_tools.packages before this PR.
• the target platform is set to osx-arm64 in pkg_defs/sherpa/conda_build_config.yaml
• the path to MacOS SDK is set to /opt/MacOSX11.0.sdk in pkg_defs/sherpa/conda_build_config.yaml. This was OK in the specific case, but in general that will change.

Originally posted by @javierggt in sot/skare3_tools#116 (comment)