gitlab-ci: disable realtime monitoring to unbreak Windows jobs

The GitLab CI runners using Windows machines have realtime monitoring
via Windows Defender enabled by default. This has just now started to
cause issues in our CI jobs using Microsoft Visual Studio:

Program 'meson.exe' failed to run: Operation did not complete successfully because the file contains a virus or
potentially unwanted softwareAt line:356 char:1
+ meson setup build --vsenv -Dperl=disabled -Dbackend_max_links=1 -Dcre ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.
At line:356 char:1
+ meson setup build --vsenv -Dperl=disabled -Dbackend_max_links=1 -Dcre ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ResourceUnavailable: (:) [], ApplicationFailedException
+ FullyQualifiedErrorId : NativeCommandFailed

The detected issue is more likely than not completely bogus, but it
breaks the jobs.

Fix the issue by disabling realtime monitoring. Besides unbreaking CI,
it also improves our build times a bit:

- Building Git goes from 26 to 22 minutes.

- Executing tests goes from ~1h for one slice of tests to ~30 minutes.

This is still painfully slow, but the issue here is that the Windows
runners on GitLab CI are quite underwhelming overall.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>