[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. Let's use `mac-beta`,
so the sole config with `check_connection=ON` will not be dropped
without replacement in the future.

Author: guitargeek

.github/workflows/root-ci-config/buildconfig/mac-beta.txt

@@ -22,6 +22,7 @@ builtin_xrootd=ON
 builtin_xxhash=ON
 builtin_zeromq=ON
 builtin_zstd=ON
+check_connection=ON
 cocoa=ON
 minuit2_omp=OFF
 x11=OFF