Allow depot_tools to update in tools/manage_deps.dart

This became a problem with: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3556588 .

Change-Id: I00d16291112d63dff190a5553805f6185ea470b9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239316
Reviewed-by: Jonas Jensen <[email protected]>
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Sigurd Meldgaard <[email protected]>

Author: sigurdm

tools/manage_deps.dart

@@ -231,6 +231,7 @@ List<String> runProcessForLines(List<String> cmd,
     cmd[0],
     cmd.skip(1).toList(),
     workingDirectory: workingDirectory,
+    environment: {'DEPOT_TOOLS_UPDATE': '0'},
   );
   printSuccessTrailer(result, onFailure);
   final output = (result.stdout as String);