Use experimental ghcup in CI

Author: hasufell

.github/workflows/test.yml

@@ -26,14 +26,26 @@ jobs:
         with:
           ghc-version: ${{ matrix.ghc }}
           run: xvfb-run -s '-screen 0 640x480x16' -a npm test
+      - name: "Install ghcup on linux"
+        if: runner.os == 'Linux'
+        run: |
+          curl -O https://downloads.haskell.org/ghcup/0.1.17.6-alpha1/x86_64-linux-ghcup-0.1.17.5
+          chmod +x x86_64-linux-ghcup-0.1.17.5
+          cp x86_64-linux-ghcup-0.1.17.5 "$(which ghcup)"
       - name: "Install `tree` for MacOs"
         run: |
+          curl -O https://downloads.haskell.org/ghcup/0.1.17.6-alpha1/x86_64-apple-darwin-ghcup-0.1.17.5
+          chmod +x x86_64-apple-darwin-ghcup-0.1.17.5
+          cp x86_64-apple-darwin-ghcup-0.1.17.5 "$(which ghcup)"
           brew update
           brew install tree
         if: runner.os == 'macOS'
       - name: "Install `tree` for Windows"
         run: |
           choco install tree
+          curl -O https://downloads.haskell.org/ghcup/0.1.17.6-alpha1/x86_64-mingw64-ghcup-0.1.17.5.exe
+          chmod +x x86_64-mingw64-ghcup-0.1.17.5.exe
+          cp x86_64-mingw64-ghcup-0.1.17.5.exe /c/ghcup/bin/ghcup.exe
         if: runner.os == 'Windows'
       # Pre-fetch HLS binaries before the tests because otherwise
       # we run into timeouts. Downloading takes longer, since we download