Makefile: Fix rebuild target to remove -F flag
Makefile: Fix rebuild target to remove -F flag

- -F flag:
  do not cache packages, cached packages will still be used for
  incremental compilation.
  note: do not 'pkg' directory.

Signed-off-by: Koichi Shiraishi <13fbd79c3d390e5d6585a21e11ff5ec1970cff0c@zchee.io>

zchee/nvim-go,zchee/nvim-go