Disable the Windows CI builds

Setting up vcpkg tries to build xz from source, which fails because the
entire xz repo has now been made private or taken down. The vcpkg team
have been advised not to switch to an alternative repo [1], so for now
our Windows builds will always fail.

Turn those builds off, so that we don't get familiar with seeing red
status markers on all PR's CI results.

[1] microsoft/vcpkg#37957 - second comment is
the vcpkg team's "We have been explicitly asked by security folks to not
change the upstream [to a different repo] for liblzma at this time."

.github/workflows/ci-main.yml

@@ -285,7 +285,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        cfg: [Debug, Release]
+        cfg: [] # All vcpkg builds require xz. Disabled while vcpkg and xz upstreams discuss recovery from the backdoor.
     env:
       CFG: ${{ matrix.cfg }}
       VCPKG_FEATURE_FLAGS: dependencygraph