💩 CI: Run Windows tests against nightly

This is to attempt to reproduce the windows-sys panic against nightly
we're seeing in busd CI:

https://github.com/dbus2/busd/actions/runs/7858111371/job/21442835251?pr=37#step:5:310

.github/workflows/rust.yml

@@ -188,7 +188,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: stable
+          toolchain: nightly
       - uses: Swatinem/rust-cache@v2
       - name: Test
         run: |