update the specification validation workflow

[devoncarew]

Update the specification validation workflow:

• always validate the specification - try to create the spec pdf
• only perform the firebase upload step if the triggering event is a PR merge

This should address the issue where automated dependabot PRs to update our github actions deps fail CI (ala #3316).

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[devoncarew]

Hmm, well, this skipped the upload phase for the PR - which is good - but also skipped it on the merge to main, which is bad.

This will need some more work -