setup_sysroot: Fix build using command:, not script:

Oops. Commit 54240d3b broke the build as I botched the directive to
use, leaving `script:` even if `command:` was now the right one as
`install` is a system command and not a local script.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>