Fix Ubuntu CI builds 2024/12/22: turn off cylc variant in skylab-dev template, shorten env path for oneapi-ifx

[climbfuji]

Summary

This PR fixes the CI build errors on the Ubuntu CI runners by:

1. Turning off the cylc variant in the skylab-dev template. We wanted to keep one CI build for cylc, even though the Skylab environment doesn't require it at the moment. But with the increasing complexity of Python dependencies, this is getting too hard. Note that Add cylc to neptune-python-env, enable in neptune-dev, disable in skylab-dev #1410 will allow us to build cylc again (because we can finally pin py-cython to a newer version). I haven't decided yet how to exactly do that in the context of CI and for releases of spack-stack, but likely it will be a separate environment (config template) altogether (deferred to Add cylc to neptune-python-env, enable in neptune-dev, disable in skylab-dev #1410).
2. Shorten the path for the Intel OneAPI ifx build. The CI build now fails, because the full path to the install location in the build environment is 2 characters too long (I don't know why it worked beforehand; maybe it was extracting the problematic packages from the buildcache instead of building them from source). Instead of oneapi-ifx, we use oneifx as the name of the build environment (it makes zero difference, other than shortening the path by four characters).

Testing

• CI

Applications affected

None

Systems affected

Ubuntu CI builds

Dependencies

n/a

Issue(s) addressed

None created - the CI build failures were first noted in #1432 (comment), therefore this PR should be merged before #1432 so that all CI tests pass.

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.