checkpkg: Avoid unnecessary use of sed(1) and cut(1)

We already use `pacman -Sddp` here which should always only return a
single package URL for regular packages. No need to extract the first
field of the last line. Also, specify "--print-format '%l'" explicitly
to make this consistent even if pacman(8)'s default value changes some
day.

Use bash parameter expansion instead of sed(1) to extract the file name
from the URL.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>