ci: Install NSIS manually for nightly builds

window-latest switched to windows-2025, where GitHub doesn't preinstall NSIS:
https://github.com/actions/runner-images/issues/12677

Let's install it manually (untested, might need follow-up commits).

The release.yml workflow uses windows-2019 (and will switch to windows-2022 in a
follow-up commit), so it is unaffected for now.

.github/workflows/nightly.yml

@@ -44,6 +44,13 @@ jobs:
         uses: actions/setup-python@v6
         with:
           python-version: "3.13"
+      - name: Install nsis
+        if: matrix.os == "windows-latest"
+        run: |
+          irm get.scoop.sh | iex
+          scoop update
+          scoop bucket add extras
+          scoop install nsis
       - name: Install dependencies
         run: |
             python -m pip install -U pip