buildman: Support building within a Python venv

The Python virtualenv tool sets up a few things in the environment,
putting its path first in the PATH environment variable and setting up
a sys.prefix different from the sys.base_prefix value.

At present buildman puts the toolchain path first in PATH so that it can
be found easily during the build. For sandbox this causes problems since
/usr/bin/gcc (for example) results in '/usr/bin' being prepended to the
PATH variable. As a result, the venv is partially disabled.

The result is that sandbox builds within a venv ignore the venv, e.g.
when looking for packages.

Correct this by detecting the venv and adding the toolchain path after
the venv path.

Signed-off-by: Simon Glass <sjg@chromium.org>