After 0ed946aa61, max-jobs setting (-j/--max-jobs)
stopped working.
The reason was that nrLocalBuilds (which compared to maxBuildJobs to figure
out whether the limit is reached or not) is not incremented yet when tryBuild
is started; So, the solution is to move the check to tryLocalBuild.
Closes https://github.com/nixos/nix/issues/3763