The fzf vim plugin wasn't working because it was making a symlink to a
directory with the full source code. This directory isn't present
anymore since the commit e95f17e272 wich
removes it because it isn't so useful for the go packages.
I fixed it by manually copying the plugin/ directory into the out
derivation, which is the only part of the source that contains the vim
plugin.
fcb4e32e9a