build system: Ensure there are no duplicate files in bindist list

Several executables inexplicably appear twice in bindist.list, which
ends up producing multiple tar file entries, consequently breaking BSD
tar during extraction. I spent a fair amount of time trying to work out
where these duplicates were coming from to no avail. Since Hadrian is
right around the corner I'm satisfied with a terrible hack: just uniq
bindist.list before producing the bindist tarball.

Test Plan: Validate

Reviewers: austin

Subscribers: rwbarton, thomie

GHC Trac Issues: #13979, #13974

Differential Revision: https://phabricator.haskell.org/D3767