ci: update Nix installation and caching

This commit updates the CI workflow to use the latest versions of the
'install-nix-action' and 'cachix-action' from
the 'cachix' organization.
It also specifies the 'nixos-unstable' channel for Nix packages.

Replaces 'determinateSystems' actions with 'cachix' actions.
Configures 'cachix' to use the 'peerswap' cache.

.github/workflows/ci.yml

@@ -47,5 +47,5 @@ jobs:
         with:
           name: peerswap
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-      - run: nix develop --profile dev-profiile 
+      - run: nix develop --profile dev-profiile
       - run: "make test-${{matrix.test-vector}}"

flake.nix

@@ -1,4 +1,10 @@
 {
+  nixConfig.extra-substituters = [
+    "https://peerswap.cachix.org"
+  ];
+  nixConfig.extra-trusted-public-keys = [
+    "peerswap.cachix.org-1:YsClAEBZtmKr/gArdsevdDTYjIQumJNn3uny1VeTxCI="
+  ];
   inputs = {
     # Pinning to revision f54322490f509985fa8be4ac9304f368bd8ab924 
     # - cln v24.02.1