Concurrent git fetches cause OOM

When multiple jobs trigger git fetches at the same time, our system ooms and dies, leaving the workspaces broken because the git lockfile is present from a dead git process.
We have the "# of executors" for the node set to 1, to avoid this sort of problem, and while this does stop it from running the actual jobs in parallel, it will still perfrom git fetches in parallel, and there doesn't seem to be any way to stop it from doing so (does anyone know a workaround?).

This has been triggered recently by the github api going down, then coming back up, causing all of our open pull requests to be rebuilt at the same time, running ~10 git fetches on large repos, which causes the whole instance to die and need a hard reboot.