utils: log envvar SNAPCRAFT_MAX_PARALLEL_BUILD_COUNT when invalid

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

No functional changes.

Minor logging improvement to log the environment variable SNAPCRAFT_MAX_PARALLEL_BUILD_COUNT when it is invalid.

[codecov-commenter]

Codecov Report

Merging #4041 (5489261) into main (ce9c24e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 5489261 differs from pull request most recent head cea11b3. Consider uploading reports for the commit cea11b3 to get more accurate results

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@           Coverage Diff           @@
##             main    #4041   +/-   ##
=======================================
  Coverage   91.68%   91.68%           
=======================================
  Files          78       78           
  Lines        5230     5231    +1     
=======================================
+ Hits         4795     4796    +1     
  Misses        435      435           

Impacted Files	Coverage Δ
snapcraft/utils.py	91.47% <100.00%> (+0.04%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more