Add support for GitHub caching of Nix builds.

.github/workflows/nix-action-master.yml

@@ -36,6 +36,8 @@ jobs:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, math-comp
         name: coq-community
+    - name: GitHub caching
+      uses: DeterminateSystems/magic-nix-cache-action@v1
     - id: stepCheck
       name: Checking presence of CI target coq
       run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\
@@ -84,6 +86,8 @@ jobs:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, math-comp
         name: coq-community
+    - name: GitHub caching
+      uses: DeterminateSystems/magic-nix-cache-action@v1
     - id: stepCheck
       name: Checking presence of CI target coq-shell
       run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\

action.nix

@@ -75,7 +75,10 @@ with builtins; with lib; let
       else [ (stepCachixUse (head reordered // {
         extraPullNames = map (v: v.name) (tail reordered);
       })) ];
-
+  stepMagicNixCache = {
+    name =  "GitHub caching";
+    uses =  "DeterminateSystems/magic-nix-cache-action@v1";
+  };
   stepCheck = { job, bundles ? [] }:
     let bundlestr = if isList bundles then "\${{ matrix.bundle }}" else bundles; in {
     name = "Checking presence of CI target ${job}";
@@ -108,6 +111,7 @@ with builtins; with lib; let
       steps = [ stepCommitToInitiallyCheckout stepCheckout1
                 stepCommitToTest stepCheckout2 stepCachixInstall ]
               ++ (stepCachixUseAll cachix)
+              ++ [ stepMagicNixCache ]
               ++ [ (stepCheck { inherit job bundles; }) ]
               ++ (map (job: stepBuild { inherit job bundles; }) jdeps)
               ++ [ (stepBuild { inherit job bundles; current = true; }) ];