.gitlab-ci.yml: Update Windows image.

The old image produced errors like

```
Downloading zlib patch from
  https://wrapdb.mesonbuild.com/v2/zlib_1.2.11-5/get_patch
A fallback URL could be specified using patch_fallback_url key in the wrap file
WrapDB connection failed to
  https://wrapdb.mesonbuild.com/v2/zlib_1.2.11-5/get_patch
with error
  <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify
   failed: certificate has expired (_ssl.c:1122)>
```