There's only one place in configure where we use `...` to execute a
command and capture the result.  Switch to $() to match the rest of
the script. This silences a shellcheck warning.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220825150703.4074125-6-peter.maydell@linaro.org