ci: Improve performance for nix jobs in CI

Defaulting to max-jobs = auto should improve build times by using more
cores.

Setting always-allow-substitutes to true should cause all nix
derivations to be cached on cachix, which should improve performance of
the MacOS job dramatically, when no rebuilds need to happen.

.github/actions/setup-nix/action.yaml

@@ -12,6 +12,10 @@ runs:
   using: composite
   steps:
     - uses: nixbuild/nix-quick-install-action@60e9c39264d4714139af3cdf15f691b19eec3530 # v28
+      with:
+        nix_conf: |-
+          always-allow-substitutes = true
+          max-jobs = auto
     - uses: cachix/cachix-action@18cf96c7c98e048e10a83abd92116114cd8504be # v14
       with:
         name: postgrest