fix: set VCPKG_ROOT for rust-sdl2 to find cargo-vcpkg libraries

cargo-vcpkg installs to target/vcpkg but rust-sdl2's vcpkg-rs crate
auto-detection doesn't reliably find it on all platforms. Set VCPKG_ROOT
explicitly to ensure consistent library discovery across platforms.

.github/workflows/build.yaml

@@ -74,9 +74,11 @@ jobs:
           sudo apt-get install -y libltdl-dev
 
       - name: Setup vcpkg
+        shell: bash
         run: |
           cargo install cargo-vcpkg
           cargo vcpkg -v build
+          echo "VCPKG_ROOT=${{ github.workspace }}/target/vcpkg" >> $GITHUB_ENV
         working-directory: pacman
 
       - name: Build