Floss: Add --print-env to build.py

--print-env prints the same environment variables used by build.py, this
is useful for setting up external tools like rust-analyzer.

The output can be `eval`-ed directly in shell or copied to the external
tool's configuration.

Bug: 176846220
Tag: #floss
Test: Manual - Ran build.py --print-env

Change-Id: I51fe9cc64237207b37ba7f8d98bd386387e735e9