Re-enable GPU builds on Linux

Author: karllessard

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
     needs: prepare
     strategy:
       matrix:
-        ext: ["", -mkl] # , -gpu, -mkl-gpu]
+        ext: ["", -mkl, -gpu, -mkl-gpu]
     steps:
       - name: Install environment
         run: |