ci: add vs-shell setup task to configure Windows env

The upstream `botan-rs` crate uses this same action in its Windows based
CI, successfully building `botan-sys` on that platform.

Without using this action, the build process fails with the error:
```
ERROR: Default compiler is msvc but could not find 'cl'; use --cc to set
```

It's possible that with effort another workaround could be used, but
it seems prudent to match what the upstream repository does and its
better to have a working setup we can iterate on than one that skips
Botan coverage entirely.

.github/workflows/ci.yml

@@ -52,6 +52,9 @@ jobs:
     - uses: actions/checkout@v4
       with:
         persist-credentials: false
+    - uses: egor-tensin/vs-shell@v2
+      with:
+        arch: amd64
     - name: Install Rust
       uses: dtolnay/rust-toolchain@master
       with: