remote_access: set $PATH when running commands on ChromiumOS devices
remote_access: set $PATH when running commands on ChromiumOS devices

Dev packages are installed in /usr/local/bin (or /usr/local/sbin).  In
some cases (e.g. a non-interactive dash shell), these paths are not
set in $PATH. This CL sets $PATH by sourcing a temporary environment
file before running the remote shell command.

Note that "scp" still does not work because we cannot source the
environment file. In most cases, rsync should be sufficient for
copying files, unless the stateful partition is corrupted.

BUG=chromium:355791
TEST=`cros deploy` on gizmo
TEST=`cros flash` on gizmo

Change-Id: I86484f9609c8a3555620bbacff4244aa99be9447
Reviewed-on: https://chromium-review.googlesource.com/191815
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite