[CI] docs, bump action versions

Author: kolloch

.github/workflows/static-docs.yml

@@ -30,10 +30,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
         - uses: actions/checkout@v4
-        - uses: cachix/install-nix-action@v20
+        - uses: cachix/install-nix-action@v27
           with:
             nix_path: nixpkgs=channel:nixos-unstable
-        - uses: cachix/cachix-action@v12
+        - uses: cachix/cachix-action@v15
           with:
             name: eigenvalue
             signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'