Remove Debug→ReleaseFast override in build.zig

zig build with no flags now builds Debug (fast compile, ~30s on VM).
zig build -Doptimize=ReleaseFast for release builds.
The override was causing all builds to take 5+ min on 2-vCPU VMs,
blocking CI and agent workflows.

Author: yevbar

build.zig

@@ -5,7 +5,7 @@ pub fn build(b: *std.Build) void {
     const enable_git_fallback = b.option(bool, "git-fallback", "Enable git CLI fallback") orelse false;
     const options = b.addOptions();
     options.addOption(bool, "enable_git_fallback", enable_git_fallback);
-    const exe_optimize: std.builtin.OptimizeMode = if (optimize == .Debug) .ReleaseFast else optimize;
+    const exe_optimize = optimize;
     const exe = b.addExecutable(.{
         .name = "ziggit",
         .root_module = b.createModule(.{