[tlbuild] mktexdir.opt install failure

[Karl Berry]

    /opt/csw/bin/ginstall: cannot create regular file
    '/path/to/texlive/Build/source/inst/texmf-dist/web2c/mktexdir.opt':
    File exists

1) I don't know of anything that has changed wrt this since the 2016
release, so I don't know how to deal with it.  At first blush, it looks
like a random race condition and you were just lucky not to hit it
before.  I'm not sure how to debug.

2) If you're going to do parallel builds, I think you should change the
install dir too (set TL_INSTALL_DEST).  I hope that will eliminate the
problem.

karl