ci: Add executable bit to malcontent-client

ci: Add executable bit to malcontent-client

For some reason, installing malcontent with buildah as a proxy, results
in the malcontent-client binary missing the execute permission, even
though manually invoking the install-meson-project.sh script results in
the correct permissions being set.

Work around this by manually setting the correct permissions on the
malcontent-client binary.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3918>