enter-env.sh doesn't re-enter

[CarlFK]

There are instructions on how to re-enter, but I think this is a problem with not leaving.

https://github.com/timvideos/litex-buildenv/wiki/HowTo-LCA2018-FPGA-Miniconf-VexRiscv-Renode#doing-the-build-of-linux

says do this and it will build.. it doesn't.

(LX P=arty C=vexriscv) juser@cnt4:~/tv/litex-buildenv$ export CPU=vexriscv CPU_VARIANT=linux PLATFORM=arty TARGET=net
(LX P=arty C=vexriscv.linux) juser@cnt4:~/tv/litex-buildenv$ source ./scripts/enter-env.sh
Already sourced this file.
(LX P=arty C=vexriscv.linux) juser@cnt4:~/tv/litex-buildenv$ ./scripts/build-linux.sh
              Platform: arty
                Target: net (default: net)
                   CPU: vexriscv.linux (default: lm32)
          Architecture: riscv32
When building Linux you should set FIRMWARE to 'linux'.

[CarlFK]

or maybe we just need to set FIRMWARE=linux

(that seemed to work)