[ci] Keep one build with check_connection=ON

Since `check_connection=ON` is the default from ROOT builds from source,
it is important to test this in the CI.

But we don't want to test this in general, because of the constant risk
that the probe file on `root.cern` can't be downloaded. So we should
test it on a platform that uses builtins that trigger the connection
check, and where sporadic build failues annoy us the least.

The platforms that use lots of builtins and will therefore trigger the
connection check forever are `windows` and `macos`. We don't have many
Windows CI configs, so we should not risk one of them.

Therefore, I suggest to use some `mac` platform. I would not use
`mac-beta`, because we should definitely not be blind to cutting-edge
macOS development. Some older - more stable - macOS release would be
better. But we also don't want to have to update the CI config too
often, so I think `mac15` is a good choice, because we still have
several years before this platform will be dropped.

Author: guitargeek

.github/workflows/root-ci-config/buildconfig/mac15.txt

@@ -23,6 +23,7 @@ builtin_xrootd=ON
 builtin_xxhash=ON
 builtin_zeromq=ON
 builtin_zstd=ON
+check_connection=ON
 cocoa=ON
 minuit2_omp=OFF
 tmva-sofie=ON