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>

packages/sgx-sdk/default.nix

@@ -18,6 +18,7 @@
 , python3
 , texinfo
 , validatePkgConfig
+, which
 , writeShellApplication
 , writeShellScript
 , writeText
@@ -91,6 +92,7 @@ stdenv.mkDerivation rec {
     python3
     texinfo
     validatePkgConfig
+    which
   ];
 
   buildInputs = [