fix: cachix/install-nix-action version in gh pages workflow

Signed-off-by: Brian McGee <brian@bmcgee.ie>

Author: brianmcgee

.github/workflows/gh-pages.yml

@@ -21,7 +21,7 @@ jobs:
             - uses: actions/checkout@v5
               with:
                   fetch-depth: 0
-            - uses: cachix/install-nix-action@V27
+            - uses: cachix/install-nix-action@V31
               with:
                   extra_nix_config: |
                       accept-flake-config = true