oe-pkgdata-util: Check if environment script is initialized

Tinfoil doesn't behave well if environment is not initialized, this check ensures a proper error log if environment is not initialized.

[YOCTO #12096]

Signed-off-by: Dorinda Bassey <dorindabassey@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>