cachix-action@v4

Fixes the intermittent /nix/var/nix/profiles/per-user permission bug on MacOS.

Author: rossabaker

.github/workflows/ci.yaml

@@ -9,7 +9,7 @@ jobs:
     steps:
     - uses: actions/checkout@v1
     - uses: cachix/install-nix-action@v6
-    - uses: cachix/cachix-action@v3
+    - uses: cachix/cachix-action@v4
       with:
         name: earnestresearch-public
         signingKey: '${{ secrets.EARNESTRESEARCH_PUBLIC_CACHIX_SIGNING_KEY }}'