gitlab-ci: migrate Windows builds away from Chocolatey

The Windows builds in GitLab CI use Chocolatey to install dependencies.
Unfortunately, Chocolatey seems to be very unreliable, which causes the
jobs to fail very regularly. This is a limitation that seems to be
somewhat known [1]:

  As an organization, you want 100% reliability (or at least that
  potential), and you may want full trust and control as well. This is
  something you can get with internally hosted packages, and you are
  unlikely to achieve from use of the Community Package Repository.

So using the Community Package Repository is kind of discouraged in case
one wants reliability. We _do_ want reliability though, and we cannot
easily switch to an enterprise license to fix this issue.

Introduce a new script that downloads and installs dependencies
directly. This has a couple of benefits:

  - We can drop our dependency on Chocolatey completely, thus improving
    reliability.

  - We can easily cache the installers.

  - We get direct control over the exact versions we install.

  - Installing dependencies is sped up from roundabout 3 minutes to 1
    minute.

[1]: https://docs.chocolatey.org/en-us/community-repository/community-packages-disclaimer/#summary

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

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