build system: fixups to --seed mechanism

* support 0x prefixed hex code for CLI seed arguments
* don't change the build summary; the printed CLI on build runner
  failure is sufficient
* use `std.crypto.random` instead of system time for entropy

Author: andrewrk

lib/build_runner.zig

@@ -202,8 +202,10 @@ pub fn main() !void {
                     std.debug.print("Expected u32 after {s}\n\n", .{arg});
                     usageAndErr(builder, false, stderr_stream);
                 };
-                seed = std.fmt.parseUnsigned(u32, next_arg, 10) catch |err| {
-                    std.debug.print("unable to parse seed '{s}' as u32: {s}", .{ next_arg, @errorName(err) });
+                seed = std.fmt.parseUnsigned(u32, next_arg, 0) catch |err| {
+                    std.debug.print("unable to parse seed '{s}' as 32-bit integer: {s}", .{
+                        next_arg, @errorName(err),
+                    });
                     process.exit(1);
                 };
             } else if (mem.eql(u8, arg, "--debug-log")) {
@@ -526,9 +528,7 @@ fn runStepNames(
             stderr.writeAll(" (disable with --summary none)") catch {};
             ttyconf.setColor(stderr, .reset) catch {};
         }
-        ttyconf.setColor(stderr, .dim) catch {};
-        stderr.writer().print("\nseed is {}\n", .{seed}) catch {};
-        ttyconf.setColor(stderr, .reset) catch {};
+        stderr.writeAll("\n") catch {};
         const failures_only = run.summary != Summary.all;
 
         // Print a fancy tree with build results.

src/main.zig

@@ -4906,6 +4906,7 @@ pub const usage_build =
     \\  --global-cache-dir [path]     Override path to global Zig cache directory
     \\  --zig-lib-dir [arg]           Override path to Zig lib directory
     \\  --build-runner [file]         Override path to build runner
+    \\  --seed [integer]              For shuffling dependency traversal order (default: random)
     \\  --fetch                       Exit after fetching dependency tree
     \\  -h, --help                    Print this help and exit
     \\
@@ -4930,8 +4931,6 @@ pub fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !voi
         var reference_trace: ?u32 = null;
         var debug_compile_errors = false;
         var fetch_only = false;
-        const micros: u32 = @truncate(@as(u64, @bitCast(std.time.microTimestamp())));
-        var seed: []const u8 = try std.fmt.allocPrint(arena, "{}", .{micros});
 
         const argv_index_exe = child_argv.items.len;
         _ = try child_argv.addOne();
@@ -4947,7 +4946,10 @@ pub fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !voi
         const argv_index_global_cache_dir = child_argv.items.len;
         _ = try child_argv.addOne();
 
-        try child_argv.appendSlice(&[_][]const u8{ "--seed", seed });
+        try child_argv.appendSlice(&.{
+            "--seed",
+            try std.fmt.allocPrint(arena, "0x{x}", .{std.crypto.random.int(u32)}),
+        });
         const argv_index_seed = child_argv.items.len - 1;
 
         {