cirrus: Cap ccache max size to 500M and max files to 20000

After #2802, depending on the task/platform, a mostly cached build on Cirrus CI
is taking 30-60seconds. Eye balling a few tasks, it is now taking longer to
download and unpack as well as check and re-upload the accumulated cache in
the beginning and end of a task.

For Debian 11, this was ~1:20 and 1:10 with a cache size of ~4.2GB. The
default size limit for ccache is 5GB, there's no limit to the number of files.

Running a fresh build on Debian 11, ccache -s indicates the actual
required cache size is ~100MB and the number of files in cache is ~2.7k.
Lower cache size to 500M and number of files to 20000, such that we don't
unnecessarily accumulate the cache and spend resources on downloading,
checking and re-uploading the cache.

    root@cirrus-ci-task-4907974120964096# ccache -s | grep -E 'files|size'
    files in cache                      2736
    cache size                          96.7 MB
    max cache size                       5.0 GB

PRs have their own cache namespace, so they won't thrash the main's
branch cache. I think main and release share their branch, so we should
not be super aggressive. Though we're probably okay with cache misses on
release once in a while.

.cirrus.yml

@@ -139,6 +139,10 @@ env:
   CCACHE_DIR: /tmp/ccache
   CCACHE_COMPRESS: 1
 
+  # Ensure reasonable ccache pruning
+  CCACHE_MAXSIZE: 500M
+  CCACHE_MAXFILES: 20000
+
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