fix(build.sh): export get_config_value and set_config_value to make ublue-update installer work

[gerblesh]

Dunno why the functions got deleted in the script, saw get_config_value was there, but wasn't exported, just fixed it up real quick. Noticed the issue when automatic updates with rpm-ostreed were still enabled when using the bling module to install ublue-update

[xynydev]

I never created such functions, I just assumed this is something you added in while PRing in the ublue-update installer, apparently not.

No idea what these do, though. Some comments would be warranted.

Edit: Ok, so these are functions for changing values in .ini or .conf style files like rpm-ostreed.conf? I don't know if this is something we'd want to be in build.sh since it's not related to reading the configuration etc. I'd say these go better inside the module's code.

[gerblesh]

originally the functions were included with the modules, I don't know how they got removed, i'll add them back in https://github.com/ublue-os/bling then

[gerblesh]

reopening this to simply delete the set_config_value function inside build.sh

[gerblesh]

nvm it was never defined, all good

[gerblesh]

New PR to fix this: blue-build/modules#37