build-remote.pl: Allow a machine to refuse a build

Before selecting a machine, build-remote.pl will try to run the
command "nix-builds-inhibited" on the machine.  If this command exists
and returns a 0 exit code, then the machine won't be used.  It's up to
the user to provide this command, but it would typically be a script
that checks whether there is enough disk space and whether the load is
not too high.

scripts/build-remote.pl.in

@@ -195,7 +195,9 @@ REQ: while (1) {
         # Connect to the selected machine.
         @sshOpts = ("-i", $machine->{sshKeys}, "-x");
         $hostName = $machine->{hostName};
-        last REQ if openSSHConnection $hostName;
+        last REQ if
+            openSSHConnection($hostName) &&
+            system("ssh $hostName @sshOpts nix-builds-inhibited >/dev/null 2>&1") != 0;
 
         warn "unable to open SSH connection to $hostName, trying other available machines...\n";
         $machine->{enabled} = 0;