.gitlab-ci.yml: Add steps to `before_script` to ensure recent CA.

Fetch current list of valid CAs from Windows Update and manually import them
to trusted datastore.  This action is required to make downloads work from
sites that need recent Let's Encrypt ISRG Root X1 certificate.

.gitlab-ci.yml

@@ -40,6 +40,9 @@ variables:
     #   -Dfoo=enabled
     #   -Dbar=disabled
   before_script:
+    # Update RootCAs in order to access to some sites.
+    - certutil -generateSSTFromWU "C:\roots.sst"
+    - Import-Certificate -CertStoreLocation "Cert:\LocalMachine\Root" "C:\roots.sst"
     # Make sure meson is up to date, so we don't need to rebuild the image
     # with each release.
     - pip3 install meson==0.59.1