build: let UnpackAll.py find 7z.exe on Windows

build: let UnpackAll.py find 7z.exe on Windows

Remove incomplete workaround - executing external_builders.scons
twice instead of just one time, from build_common\SConscript.
UnpackAll.py was not able to find the path to 7z.exe when using that
workaround, and SCons was not able to unpack Gradle.

Replace that workaround by fixing SCons's PATH earlier, before
UnpackAll.py starts - similarly to the way build_common\SConscript
handles the PATH on non-Windows platforms.

[Philippe Coval]

Conflicts:
	build_common/SConscript

Bug: https://jira.iotivity.org/browse/IOT-1745
Change-Id: Ic90337c6c34556043017f2c55afbc1edcac402ef
Signed-off-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21063
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Mats Wichmann <mats@linux.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Signed-off-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21343