chore: pr-release: fix MESSAGE calculation

[nomeata]

previously there was a logic bug where the check if a std tag exists would override the result of whether a mathlib tag exists (both wrote MESSAGE in both cases).

Now we only check for the mathlib tag if we found the std tag.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• 💥 Mathlib branch lean-pr-testing-3631 build failed against this PR. (2024-03-07 11:18:59) View Log
• 💥 Mathlib branch lean-pr-testing-3631 build failed against this PR. (2024-03-07 12:05:39) View Log

[kim-em]

Sorry, missed this, and have already merged #3700.