CI: include zlib devel package for the mingw cross tests

After a recent CI image builder update successfully ran the tests, but
didn't run the cross tests, it updated the image that then got used by
the regular CI cross tests. Somehow this resulted in a bunch of tests
now failing because zlib could not be picked up. We probably dropped a
transitive dependency somewhere. Anyway, it's correct to explicitly
specify it if we need it.

ci/ciimage/ubuntu-rolling/install.sh

@@ -15,7 +15,7 @@ pkgs=(
   libomp-dev
   llvm lcov
   dub ldc
-  mingw-w64 mingw-w64-tools
+  mingw-w64 mingw-w64-tools libz-mingw-w64-dev
   libclang-dev
   libgcrypt20-dev
   libgpgme-dev