luci-base: runtime.uc: avoid Lua not installed exeption from trycompile()

Make sure to request loading the Lua bridge as optional when initializing
the Lua VM context from trycompile() in order to not raise a fatal exception
in case the Lua runtime support is not present.

Ref: https://forum.openwrt.org/t/x/141426
Signed-off-by: Jo-Philipp Wich <jo@mein.io>