chore: Migrate from cachix to Attic in nix and github workflows

- Updated nix config to use Attic substituter and trusted public key
- Modified github workflows to use Attic cache instead of Cachix
- Removed the now unnecessary cachix config settings and Cachix workflow actions
- Update `flake.lock`
- Run on our own CI runners

Signed-off-by: Harald Hoyer <harald@matterlabs.dev>

flake.lock

@@ -55,11 +55,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1717281328,
-        "narHash": "sha256-evZPzpf59oNcDUXxh2GHcxHkTEG4fjae2ytWP85jXRo=",
+        "lastModified": 1719707984,
+        "narHash": "sha256-RoxIr/fbndtuKqulGvNCcuzC6KdAib85Q8gXnjzA1dw=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "b3b2b28c1daa04fe2ae47c21bb76fd226eac4ca1",
+        "rev": "7dca15289a1c2990efbe4680f0923ce14139b042",
         "type": "github"
       },
       "original": {
@@ -84,11 +84,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1716675292,
-        "narHash": "sha256-7TFvVE4HR/b65/0AAhewYHEJzUXxIEJn82ow5bCkrDo=",
+        "lastModified": 1719005984,
+        "narHash": "sha256-mpFl3Jv4fKnn+5znYXG6SsBjfXHJdRG5FEqNSPx0GLA=",
         "owner": "snowfallorg",
         "repo": "lib",
-        "rev": "5d6e9f235735393c28e1145bec919610b172a20f",
+        "rev": "c6238c83de101729c5de3a29586ba166a9a65622",
         "type": "github"
       },
       "original": {