build.plat: bypass tool detection if NMIGEN_*_env is set.

build.plat: bypass tool detection if NMIGEN_*_env is set.

It's not practical to detect tools within the toolchain environment
for various reasons, so just assume the tools are there if the user
says they are.

Before this commit, the tools would be searched outside the toolchain
environment, which of course would always fail for Vivado, ISE, etc.