CI: Actually use the requested compiler

We were always testing with dmd-latest.

Author: Geod24

.github/workflows/ci.yml

@@ -15,6 +15,8 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - uses: dlang-community/setup-dlang@v1
+      with:
+        compiler: ${{ matrix.dc }}
 
     - name: 'Build & Test'
       shell: bash