aws: improve check for aws_completer - unhardcode path (#9123)

Closes #9122

The current check is assuming that the aws_completer is installed
globally, it then runs the command without that path

I have aws_completer in ~/.local/bin/aws_completer

This changes the check to see if the command exists instead of
concerning itself with where it in installed. This test matches the
behavior of just running the command.

plugins/aws/aws.plugin.zsh

@@ -63,7 +63,7 @@ fi
 # Load awscli completions
 
 # AWS CLI v2 comes with its own autocompletion. Check if that is there, otherwise fall back
-if [[ -x /usr/local/bin/aws_completer ]]; then
+if command -v aws_completer &> /dev/null; then
   autoload -Uz bashcompinit && bashcompinit
   complete -C aws_completer aws
 else