build runner: clean up tmp dirs

lib/compiler/build_runner.zig

@@ -784,6 +784,9 @@ fn runStepNames(
     var pending_count: usize = 0;
     var total_compile_errors: usize = 0;
 
+    var cleanup_task = io.async(cleanTmpFiles, .{ io, step_stack.keys() });
+    defer cleanup_task.await(io);
+
     for (step_stack.keys()) |s| {
         test_pass_count += s.test_results.passCount();
         test_skip_count += s.test_results.skip_count;
@@ -1849,3 +1852,14 @@ fn initStdoutWriter(io: Io) *Writer {
     stdout_writer_allocation = Io.File.stdout().writerStreaming(io, &stdio_buffer_allocation);
     return &stdout_writer_allocation.interface;
 }
+
+fn cleanTmpFiles(io: Io, steps: []const *Step) void {
+    for (steps) |step| {
+        const wf = step.cast(std.Build.Step.WriteFile) orelse continue;
+        if (wf.mode != .tmp) continue;
+        const path = wf.generated_directory.path orelse continue;
+        Io.Dir.cwd().deleteTree(io, path) catch |err| {
+            std.log.warn("failed to delete {s}: {t}", .{ path, err });
+        };
+    }
+}