Fix rtools40 build (#5326)

Fix rtools40 build. vcpkg-tool released a change which no longer treated environment variables as case-insensitive, then removed the variables -- breaking tools which depend on those variables.

Explanation: microsoft/vcpkg#41199 (comment)

x-ref:
```
- microsoft/vcpkg-tool#1501
- via cameleon-rs/cameleon#193
- via microsoft/vcpkg#41154
```
---
TYPE: NO_HISTORY
DESC: Fix rtools40 build.

.github/workflows/build-rtools40.yml

@@ -43,6 +43,9 @@ jobs:
         uses: r-windows/install-rtools@master
       - name: Building TileDB with Rtools40
         run: |
+          export SystemDrive="$SYSTEMDRIVE"
+          export SystemRoot="$SYSTEMROOT"
+          export windir="$WINDIR"
           cd ${GITHUB_WORKSPACE}/.github/workflows/mingw-w64-tiledb
           pacman -Sy
           makepkg-mingw --noconfirm --syncdeps