gitlab-ci: restrict maximum number of link jobs on Windows

gitlab-ci: restrict maximum number of link jobs on Windows

The hosted Windows runners on GitLab.com only have 7.5GB of RAM. Given
that "link.exe" provided by Microsoft Visual Studio is multi-threaded by
itself already and thus quite memory hungry this can quickly lead to
memory starvation, out-of-memory situations and thus failed CI jobs.

Fix the issue by limiting the number of concurrent linker jobs. The same
issue hasn't been observed on GitHub Actions yet, probably because it
got more than twice the amount of RAM with 16GB.

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