oeqa/commands: Ensure sync can be found regardless of PATH

Avoid command not found errors shown in selftest logs due to changes to PATH
settings which also risks intermittent problems due to IO load.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 40bcae01)
Signed-off-by: Steve Sakoman <steve@sakoman.com>