gs-details-page: Add list-button style to Cancel button

This makes it flatter, which [looks
nicer](GNOME/gnome-software!910 (comment 1247639)).

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1351
3 jobs for 1351-install-buttons in 4 minutes and 32 seconds (queued for 5 seconds)