build: add configurePhase for urbit derivation
build: add configurePhase for urbit derivation

46a438f introduced use of the stdenv generic builder instead of an
explicit builder.sh script.  This was reverted in 1daaf041a as it proved
to cause problems when building on some Linux variants.

This fixes the problem encountered on Linux by calling the standard
configure script with bash in an explicit configurePhase.

jfranklin9000/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit