build: more Zig 0.15.2 updates (#9217)

- update nixpkgs now that Zig 0.15.2 is available in nixpkgs
- drop hack that worked around compile failures on systems with more
than 32 cores
- enforce patch version of Zig

build.zig.zon

@@ -3,7 +3,7 @@
     .version = "1.2.1",
     .paths = .{""},
     .fingerprint = 0x64407a2a0b4147e5,
-    .minimum_zig_version = "0.15.1",
+    .minimum_zig_version = "0.15.2",
     .dependencies = .{
         // Zig libs