Switch to hermetic OpenSSH

The Bazel Central Registry (BCR) has @openssh//:{ssh,scp}.

We should use those to make our execution hermetic.

It would require adding those to the tools for the labgrid_config_toolchains and then injecting the binaries into the LabGrid configuration so that the SSHDriver picks up the hermetic implementations.

This may require upstream work as currently the SSHDriver uses ssh found on the PATH. It should pick SSHDriver from the configured tools which seems like a straight-forward piece of work:

ssh = self.target.env.config.get_tool("ssh")
cmd = [ssh, ...]  # rather than `["ssh", ...]