hack: Apptainer-only backend

[acfoltzer]

Quick hack at an Apptainer-only backend. Unlike the LSF backend, this only works if apptainer is available to execute locally. Use the backend type "apptainer" instead of "lsf_apptainer", and remove the LSF queue configs to get it configured.

For all contributors:

• You have added a few sentences describing the PR here.
• You have added yourself or the appropriate individual as the assignee.
• You have added at least one relevant code reviewer to the PR.
• Your code builds clean without any errors or warnings.
• You have added tests (when appropriate).
• You have added an entry in the CHANGELOG (when appropriate).
• You have updated the README or other documentation to account for these changes (when appropriate).
• You have either (a) made a PR to the next branch in the sprocket.bio repository [if you are a Sprocket team member] or (b) have suggested any changes you think need to be made to sprocket.bio in the description of your PR [if you are an external contributor].

For PRs containing lint rule changes:

• You have updated any and all effected entries within RULES.md.
• You have added a test case in crates/wdl-lint/tests/lints that covers every
  possible diagnostic emitted for the rule within the file where the rule
  is implemented.

[peterhuene]

@acfoltzer can this be closed now that the hackathon is over?

[acfoltzer]

Yep, there's not really much here that isn't present back on main.