Set fallback for binary caches

This makes builds not fail when my own binary cache is offline for some reason

homes/julian/hm-standalone-config.nix

@@ -39,5 +39,8 @@
     ];
 
     # nix.settings. # warn-dirty = false; # TODO: do I want this
+    #
+    # Ensure we can still build when missing-server is not accessible
+    fallback = true;
   };
 }

hosts/common/optional/binarycaches.nix

@@ -21,5 +21,8 @@
       "root"
       "@wheel"
     ]; # needed for devenv to add custom caches
+
+    # Ensure we can still build when missing-server is not accessible
+    fallback = true;
   };
 }