buildtest.py: Use ccache

[mattst88]

Reduces the time to run ./buildtest.py on my 8-core laptop from 25 minutes to 7 minutes.

 # Before, without ccache
$ time ./buildtest.py
[...]
./buildtest.py  5844.08s user 755.75s system 434% cpu 25:17.99 total

 # After, with ccache, empty cache.
$ time ./buildtest.py
[...]
./buildtest.py  1620.97s user 472.66s system 482% cpu 7:13.54 total

The ccache -s status after building once from an empty cache shows just how much duplicated work is performed: 83.00% cache hits!

$ ccache -s
Cacheable calls:    12538 / 22769 (55.07%)
  Hits:             10407 / 12538 (83.00%)
    Direct:          9093 / 10407 (87.37%)
    Preprocessed:    1314 / 10407 (12.63%)
  Misses:            2131 / 12538 (17.00%)
Uncacheable calls:  10231 / 22769 (44.93%)
Local storage:
  Cache size (GiB):   0.1 /   5.0 ( 1.97%)
  Hits:             10407 / 12538 (83.00%)
  Misses:            2131 / 12538 (17.00%)

---

Thank you for contributing your time to the Mosquitto project!

Before you go any further, please note that we cannot accept contributions if
you haven't signed the Eclipse Contributor Agreement.
If you aren't able to do that, or just don't want to, please describe your bug
fix/feature change in an issue. For simple bug fixes it is can be just as easy
for us to be told about the problem and then go fix it directly.

Then please check the following list of things we ask for in your pull request:

• Have you signed the Eclipse Contributor Agreement, using the same email address as you used in your commits?
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• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run make test with your changes locally?

---

[mattst88]

Noticed this opportunity when looking at the CI results for #3525 -- the build variants workflow took 44 minutes. The next longest was 20 minutes, and everything else was < 10 minutes.

[mattst88]

"Build variants" finished in 13 minutes :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ralight]

Lovely. The run time of this has been bugging me for a while but I didn't want to give it up.