feat: support apptainer (in particular avoid warnings)

[wdconinc]

Briefly, what does this PR introduce?

Apptainer warns when SINGULARITY_* and APPTAINER_* variables are different. This will make the user experience smoother with fewer warnings.

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.

[Copilot]

Pull request overview

This PR adds support for Apptainer to prevent warnings when SINGULARITY_* and APPTAINER_* environment variables differ. The changes prioritize Apptainer over Singularity when both are available, set corresponding APPTAINER_* environment variables alongside existing SINGULARITY_* variables, and update user-facing messages and documentation to reflect support for both tools.

Changes:

• Added APPTAINER_TMPDIR and APPTAINER_BINDPATH environment variable exports
• Modified binary detection logic to prefer Apptainer over Singularity
• Updated error messages, help text, and comments to reference Apptainer alongside Singularity

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