makeBootstrapTools: set schedulingPriority
makeBootstrapTools: set schedulingPriority

Increase schedulingPriority of the bootstrap tools to unblock the
nixpkgs-unstable channel.

The channel is repeatedly blocked by the makeBootstrapTools job for
aarch64. The cause is lack of resources.

By increasing the priority, it should become the first job Hydra would
build, allowing the channel to advance quicker. Of course, it does mean
that while the channel advances, nothing else has been built.

This should be a temporary solution until we have more capacity for
aarch64.

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