asdf: fix completions if ASDF_DIR is already exported (#8538)

plugins/asdf/asdf.plugin.zsh

@@ -3,7 +3,7 @@ ASDF_DIR="${ASDF_DIR:-$HOME/.asdf}"
 ASDF_COMPLETIONS="$ASDF_DIR/completions"
 
 # If not found, check for Homebrew package
-if [[ ! -f "$ASDF_DIR/asdf.sh" ]] && (( $+commands[brew] )); then
+if [[ ! -f "$ASDF_DIR/asdf.sh" || ! -f "$ASDF_COMPLETIONS/asdf.bash" ]] && (( $+commands[brew] )); then
    ASDF_DIR="$(brew --prefix asdf)"
    ASDF_COMPLETIONS="$ASDF_DIR/etc/bash_completion.d"
 fi