Avoid reinstalling Gnuplot on OSX in CI builds

[bheisler]

I don't know anything about OSX myself, but I do know that installing Gnuplot on the OSX CI builds takes a really long time. The script is probably doing something dumb, but if there are any OSX experts out there who wouldn't mind helping, that would be appreciated.

[neeldug]

From what I've seen in the CI, the version of Homebrew packaged with the CI seems to be really out of date and so updates initially, otherwise it seems fine. However, the autoupdates can be disabled, so maybe that'll reduce the CI timing issue.