fix: config issues when user hasn't set FZF_DEFAULT_OPTS

The important fzf options that I ensured would not be overridden by a
user's FZF_DEFAULT_OPTS were not being applied if the environment
variable was unset. The important options are:

    --no-select-1 --ansi --no-multi

This caused ansi escape sequences to not be handled properly, leading to
garbled text. Interestingly, setting the env var to any value would fix
the issue. For example:

    $ FZF_DEFAULT_OPTS="" gh fzf issue

Shells are weird.

gh-fzf

@@ -107,7 +107,7 @@ fi
 
 # The following fzf options are shared by all commands. They are prepended to
 # the FZF_DEFAULT_OPTS environment variable so they can be overridden by users.
-FZF_DEFAULT_OPTS='
+export FZF_DEFAULT_OPTS='
 --no-hscroll --no-exit-0 --header-lines=1
 --preview-window="right,50%,wrap,~2,<60(down,40%,wrap)"
 --delimiter="  "