Install alt ghc in a separate step

Author: jneira

.azure/windows-stack.yml

@@ -43,8 +43,7 @@ jobs:
       curl -sSkL http://www.stackage.org/stack/windows-x86_64 -o /usr/bin/stack.zip
       unzip -o /usr/bin/stack.zip -d /usr/bin/
       mkdir -p "$STACK_ROOT"
-      GHC_VERSION=${YAML_FILE:6:5}
-      echo "local-programs-path: D:/bin/stack-$GHC_VERSION" > "$STACK_ROOT/config.yaml"
+      echo "local-programs-path: D:/bin/stack" > "$STACK_ROOT/config.yaml"
     displayName: Install stack
   - bash: |
       source .azure/windows-stack.bashrc
@@ -92,6 +91,7 @@ jobs:
     displayName: "Install Runtime Test-Dependencies: z3"
   - bash: |
       source .azure/windows-stack.bashrc
+      stack setup --resolver=lts-13.20 
       stack install --resolver=lts-13.20 liquid-fixpoint-0.8.0.2 liquidhaskell-0.8.6.2
     displayName: "Install Runtime Test-Dependencies: liquidhaskell"
   - bash: |