hosts/hydra: use secrets for nix-cache

hosts/hydra-1/configuration.nix

@@ -84,7 +84,7 @@
     enable = true;
     port = 5005;
     bindAddress = "127.0.0.1";
-    secretKeyFile = "/var/src/secrets/nix-serve/secret-key.key";
+    secretKeyFile = config.age.secrets.nix-cache-key.path;
     # Workaround https://github.com/NixOS/nix/issues/7704#issuecomment-1409480909
     package = pkgs.nix-serve-ng.override {
       nix = pkgs.nixVersions.nix_2_12;

hosts/hydra-1/secrets/nix-cache-key.age

@@ -0,0 +1,9 @@
+age-encryption.org/v1
+-> ssh-ed25519 HwR33w XAZWRRep4zdbZFwRvWGa9yyLe44TlOGIMwtLo/WZXyw
+G8Rv8Oqbvx5qYr8ZrQlmIyhYqUG6aJArfhMoqu4U/Uc
+-> ssh-ed25519 UgSomQ sP6Z3WzhBXDaDfsSNHY7p/vV20/eytII2VhO6+vUAhE
+q6+FH+k04jG0417KvY8cHnNeb8hEzEtxX+hLmKWqvdk
+-> *A>\V=-grease
+wpRswQ45ywiN+jit7kj5p0s2a+WloUueeu5E+Nn41WOMNQLJ3RaJqxRk
+--- TVK8uHcE2tn7qfDF2oAGb24N6a670Fc+QVU7agmJYNo
+_	a<><61>	<09>-<2D><>'<0E>ӳ<0E><>AbS<62>	<09>X<EFBFBD>2<EFBFBD>$<24>\<5C>M<EFBFBD><4D>ž<EFBFBD><C5BE><0E><EFBFBD>#6<>wP<><50>Ʌ<18><>'<27>GK<47><4B>&<18><0F>|<7C><><EFBFBD>7<EFBFBD>P<EFBFBD>Ռ<EFBFBD>-v<><76><EFBFBD>_Sb&^Yұb<D2B1>(K<><4B><04>9o<39><6F><EFBFBD><EFBFBD><EFBFBD>߉{C<><43>m<EFBFBD>F0w
<19><><EFBFBD>ƣ-<0C>9<EFBFBD><5	L̥<4C>

hosts/hydra-1/secrets/nix-cache-key.pub

@@ -0,0 +1 @@
+nix-cache.clerie.de:bAt1GJTS9BOTcXFWj3nURrSlcjqikCev9yDvqArMP5g=