luci.mk: fix compatibility with GNU Make 4.2

GNU Make version 4.2 chokes on `#` characters embedded in variable
substitutions, leading to the following error:

    ../../luci.mk:114: *** unterminated call to function 'if': missing ')'.  Stop.

Avoid this problem by replacing variable substitutions with Git branch
format arguments.

Ref: https://github.com/openwrt/luci/pull/5976#issuecomment-1289935106
Signed-off-by: Jo-Philipp Wich <jo@mein.io>