rust: support for Windows

The initial patch series that introduced Rust into the core of Git only
cared about macOS and Linux. This specifically leaves out Windows, which
indeed fails to build right now due to two issues:

  - The Rust runtime requires `GetUserProfileDirectoryW()`, but we don't
    link against "userenv.dll".

  - The path of the Rust library built on Windows is different than on
    most other systems systems.

Fix both of these issues to support Windows.

Note that this commit fixes the Meson-based job in GitHub's CI. Meson
auto-detects the availability of Rust, and as the Windows runner has
Rust installed by default it already enabled Rust support there. But due
to the above issues that job fails consistently.

Install Rust on GitLab CI, as well, to improve test coverage there.

Based-on-patch-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Based-on-patch-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -1707,6 +1707,10 @@ rust_option = get_option('rust').disable_auto_if(not cargo.found())
 if rust_option.allowed()
   subdir('src')
   libgit_c_args += '-DWITH_RUST'
+
+  if host_machine.system() == 'windows'
+    libgit_dependencies += compiler.find_library('userenv')
+  endif
 else
   libgit_sources += [
     'varint.c',