Fix check tlmgr --version for macOS

[timur-harin]

@yegor256 When try to complete tlmgr step on macOS, I faced with issue

/Users/timur/develop/cam/help/assert-tool.sh: line 35: tlmgr: command not found
I can't continue, because 'tlmgr' command line tool is not available.

This is because PATH export executes only if Linux

PATH=$PATH:$("${LOCAL}/help/texlive-bin.sh")