attempt to cache /nix in Nix Shell Github Action

cachix/install-nix-action#56 (comment)

.github/workflows/nix-shell.yml

@@ -13,6 +13,30 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/[email protected]
+    # https://github.com/cachix/install-nix-action/issues/56
+    - run: |
+        # Create with liberal rights, otherwise cache action will complain
+        # about permission errors.
+        sudo mkdir -p /nix/store
+        sudo chmod -R 777 /nix
+
+    - name: Cache nix env take N+1
+      uses: actions/cache@v2
+      with:
+        path: |
+          # See https://github.com/actions/cache/pull/726
+          /nix/store/**
+          # Missing something?
+          /nix/var/nix/*/*
+          /nix/var/nix/db/*
+          /nix/var/nix/db/*/**
+          !/nix/var/nix/daemon-socket/socket
+          !/nix/var/nix/userpool/*
+          !/nix/var/nix/gc.lock
+          !/nix/var/nix/db/big-lock
+          !/nix/var/nix/db/reserved
+        key: ${{ runner.os }}-nix-store
+
     - uses: cachix/install-nix-action@v16
       with:
         nix_path: nixpkgs=channel:nixos-21.11