README: Make $GOPATHs more explicit, use sudo group
README: Make $GOPATHs more explicit, use sudo group

Add $GOPATH prefix to binaries that are run to import images in the readme.
In addition use the 'sudo' group as a default.

Signed-off-by: Chris J Arges <45f72cfd9e326d354b075e92183b7207e3b42b62@canonical.com>

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