Allow passing environment variables to `cmd_args`

It would be nice if `cmd_args` had an `env` attribute like many of the prelude rules support. Is there a reason for not implementing it, or has it just not been an issue before?

In case this is an XY situation, the actual problem is that we'd like to `LD_PRELOAD` our C++ compiler to use jemalloc. We can use a wrapper script for that, but it makes `buck2 log what-ran` quite a bit less friendly.