libcanfestival: use http download instead of hg checkout

libcanfestival: use http download instead of hg checkout

This was suggested by @hnyman because hg is not a prerequisite in
LEDE or Openwrt, so using that should be avoided, if possible.

The tricky part is that the filename is appended to the URL,
thus we construct/use an URL where the appended filename does not hurt.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>