build(nix): filter src to reduce useless rebuilds

This in particular speeds up the dev/run/debug cycle for writing more
tests. Without this, the actual bcachefs-tools package will be rebuilt
by Nix all the time because of changes to files that are tracked by git.
Even when it's changes in parts that aren't relevant to that package
build such as `checks/*`.

Signed-off-by: Thomas Mühlbacher <tmuehlbacher@posteo.net>

flake.nix

@@ -58,6 +58,7 @@
         }:
         let
           inherit (builtins) readFile split;
+          inherit (lib) fileset;
           inherit (lib.lists) findFirst;
           inherit (lib.strings) hasPrefix removePrefix substring;
 
@@ -74,7 +75,17 @@
 
           commonArgs = {
             inherit version;
-            src = self;
+            src = fileset.toSource {
+              root = ./.;
+
+              fileset = fileset.difference (fileset.gitTracked ./.) (
+                fileset.unions [
+                  ./checks
+                  ./doc
+                  ./tests
+                ]
+              );
+            };
 
             env = {
               PKG_CONFIG_SYSTEMD_SYSTEMDSYSTEMUNITDIR = "${placeholder "out"}/lib/systemd/system";