frontend: fix use of config/environment[.binary]

the environment variables when held in a variable need to be used via
`env` otherwise you get command not found errors.

example:
```
$ echo "AA=aa" > config/environment
$ lb config
/usr/bin/lb: 83: AA=aa: not found
$ ENV="AA=aa"
$ ${ENV} true
bash: AA=aa: command not found
$ "${ENV}" true
bash: AA=aa: command not found
$ env ${ENV} true
```

Gbp-Dch: Short

frontend/lb

@@ -80,4 +80,5 @@ if [ "${_COLOR_OUT}" = "true" ]; then
 	_CMD_LABEL="${CYAN}${_CMD_LABEL}${NO_COLOR}"
 fi
 Echo "[%s] ${_CMD_LABEL} %s" "$(date +'%F %T')" "$(echo ${@})"
-${ENV} exec "${SCRIPT}" "${@}"
+
+exec /usr/bin/env ${ENV} "${SCRIPT}" "${@}"