tweak build-toolchains.sh to work with conda env
tweak build-toolchains.sh to work with conda env

* more env vars need to be cleared when using the RISCV cross compiler
* use LD_GOLD to link QEMU because QEMU's LDFLAGS_NOPIC uses incorrect format for the version
  of bfd ld

ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard