Fix a couple of issues with envkernel workflow

• abuild depends on gcc but it doesn't get installed if we're using envkernel.sh --gcc6. The gcc binary is only used by abuild to find the host arch. We can set the CBUILD env var so abuild doesn't need to use gcc to detect it.
• envkernel has been broken for kernels that don't set a kernel build output directory. It appears to be an error in the original merge commit. When I refactored some things during the code review, I didn't test this case.