tools/format-cpp: make shellcheck happy again

This patch:

* Checks the `version` variable assignment directly to avoid relying on
  `$?`, therefore removing the `shellcheck disable` comment.

* Makes `formatter` an array to satisfy SC2086, therefore removing the
  `shellcheck disable` comment.

* Quotes `$(nproc)` to satisfy SC2068.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib7c5fe530faad12704ee8b91ae55a6bfcb1aee5c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11142
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>