Makefile: use simple variables
Makefile: use simple variables

There is no reason to run go env at variable expansion time, run it at
initialization time instead.
Also remove extra quoting for SHELL, as those are kept verbatim in the $(shell)
calls, which can lead to the attempted execution of a "/bin/bash" process
depending on the system, leading to errors like:

  make: "/bin/bash": Command not found

golang/dep,golang/dep,golang/dep