The hydra-queue-runner opens a connection to the builder. If the builder is 'localhost' it starts nix-store, otherwise it starts 'ssh'.

Currently, if the hydra-queue-runner can not start nix-store (not in the PATH for instance), the error message is:

cannot connect to ‘localhost’: error: cannot start ssh: No such file or directory

This is not useful since ssh is actually not started:/

With this patch the error message is now:

cannot connect to ‘localhost’: error: cannot start nix-store: No such file or directory