go.bbclass: disable the use of the default configuration file

We need to disable the use the default configuration file. This is
to ensure that user settings do not mess things up when building go
recipes.

For example, if I set 'GOBIN=./relative/path' in $HOME/.config/go/env,
then go-runtime fails to build with error like below:

  cannot install, GOBIN must be an absolute path

According to `go help environment',
"""
Setting GOENV=off in the environment disables the use of the default
configuration file.
"""

We can explicitly disable the configuration file by setting GOENV to off.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 711b4174)
Signed-off-by: Steve Sakoman <steve@sakoman.com>