Previously get_idf_build_env didn't include the default environment
(os.environ) in its output, so the environment only contained the
variables returned by "idf_tools.py export".
If PATH already contains all the right paths, "idf_tools.py export"
doesn't return the PATH variable.
Therefore the environment returned by get_idf_build_env was usually
incomplete. Fix by combining the output of "idf_tools.py export" with
os.environ.
