Add a workflow vulnerability scanner

[dkfellows]

This is work in progress...

This adds a workflow vulnerability scanner and fixes the issues it has found. Still to do:

• Create scanner workflow
• Fix issues found in reusable workflows
• Fix issues found in actions
• Test with hpcflow test of hpcflow/github-actions@scan branch hpcflow#1022
• Test with matflow test of hpcflow/github-actions@scan branch matflow#568

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[dkfellows]

All remaining issues are... arguably not problems, given that the suggested fixes make things worse in my eyes in the context of what is being done.

[dkfellows]

All remaining issues are ones I decline to fix. We need to test.

[dkfellows]

Tests with hpcflow now pass. I think the other workflows should be OK too (the main tricky bit is that the comparison action is using powershell, which has totally different syntax once you don't use direct substitution).

[dkfellows]

Test with matflow passes as well.