envsetup: Use GNU sed commands if GNU sed is detected

The current check only checks for Linux, but many of us
use GNU sed on darwin as well, which means the non-Linux
command will fail, so read the sed version and use the GNU
command if GNU is found in the version string.

Change-Id: I7eefd0309751b069eef5e5e2b907d3d4f38f1638