Restore Nix caching

.gitlab-ci.yml

@@ -86,7 +86,17 @@ build-8.8:
     paths:
       - store.nar
 
+  before_script:
+    - |
+      if [ -e store.nar ]; then
+        echo "Extracting cached Nix store..."
+        nix-store --import -vv < store.nar || echo "invalid cache"
+      else
+        echo "No cache found"
+      fi
+
   script:
+      # Install GHC
     - echo "Bindist tarball is $GHC_TARBALL"
     - |
       nix build \
@@ -96,6 +106,14 @@ build-8.8:
       ghcHEAD
     - export GHC=`pwd`/ghc/bin/ghc
     - rm -Rf $HOME/.cabal/packages/local ci/run
+      # Build CI executable
+    - nix build -f ./ci -j$CPUS --no-build-output
+    - |
+      nix-store --export \
+        $(nix-store -qR --include-outputs \
+          $(nix-instantiate --quiet scripts/build-all.nix --arg bindistTarball $GHC_TARBALL -A buildDepends -A ghc)) \
+      > store.nar
+      # Test it
     - nix run -f ./ci -c run-ci
 
   after_script: