fix(init): add cached completions directory only once (#12551)

Double quotes within the subscript test are interpreted literally, making the
test never match. On subsequent runs, the `$ZSH_CACHE_DIR/completions` directory
is always prepended to `$fpath`, even if it is already there. This commit fixes
that.

Fixes #12551

Co-authored-by: Adem Simsek <adem@simsek.dev>