modules/nixos: change some nix.conf settings

Reduce `connect-timeout`, `download-attempts`, and disable `preallocate-contents`. At first these may all seem like bad choices, but they have their uses. Currently, `connect-timeout` can cause Nix to hang when it fails to connect since it waits for the timeout to be reached. This isn't for just one or two things, this is for _all_ network IO that occurs from Nix, which can block on a lot of unnecessary things, such as trying to update the local narinfo db when it doesn't do anything for our invocation. `download-attempts` follows in tandem to this issue, by causing further blocks on unnecessary tasks, and as a result we want to be slightly less forgiving under low network situations.

Disabling `preallocate-contents` is actually very important because it breaks BTRFS COW functionality and compression. Furthermore, the feature itself isn't actually that important on a modern system, so doing it ahead-of-time isn't bringing any major benefits. The negatives here outweigh the non-existent positives, so it has been removed.

modules/nixos/programs/nix/module.nix

@@ -86,20 +86,20 @@ in {
 
           builders-use-substitutes = true;
 
-          connect-timeout = 15;
+          connect-timeout = 5;
           cores = 2;
 
           debugger-on-trace = true;
           # debugger-on-warn = true;
-          download-attempts = 3;
+          download-attempts = 2;
           eval-cache = true;
 
           # It's useful to know when a substitute is failing!
           fallback = false;
 
           # Improve the chances of the store surviving a random crash.
           fsync-metadata = true;
-          # fsync-store-paths = true;
+          # fsync-store-paths = true; TODO: on next Lix release
 
           # Keeping these is very useful for development.
           keep-build-log = true;
@@ -110,7 +110,7 @@ in {
           max-jobs = "auto";
           min-free = 10 * 1024 * 1024 * 1024;
 
-          preallocate-contents = true;
+          preallocate-contents = false; # Unnecessary on modern I/O
 
           # Never allow a non-sandboxed build
           sandbox-fallback = false;