client: fix base path location deduction when sourced

- Since these files are only ever sourced, deduction must be relative to
  `BASH_SOURCE` (or else realpath will break)

- Fixes client-side `plugins` completion
  * realpath breakage was introduced in #301

client/src/docker/lib/internal/lib_env.bash

@@ -90,7 +90,7 @@ function lib_env::env()
   # Environment location
   # NOTE: keep aligned with gen.bash
   local _env_dir
-  _env_dir="$(dirname "$(realpath -s $0)" | rev | cut -d'/' -f3- | rev)/conf.d/${global_tag_dir}/env"
+  _env_dir="$(dirname $(realpath -s "$BASH_SOURCE" | rev | cut -d'/' -f6- | rev))/conf.d/${global_tag_dir}/env"
   local _env_file="${_env_dir}/${global_conf_filename}"
 
   # shellcheck source=/dev/null