jjui: remove env_set to support repo-level configuration overrides

[abgox]

• In the next version, if no environment variable is set, jjui can use repo-level configurations to override global configurations: add repo config file which overrides global config.toml idursun/jjui#609
• Therefore, env_set should be removed from jjui manifest.
• However, due to the limitations of the official manifest, it cannot be handled as easily as abyss: IbrahimDursun.jjui: remove env_set to support repo-level configuration overrides abgox/abyss#174
• If we remove env_set directly, the configurations of existing installed users will remain in the persist directory and become invalid in future updates.
• Alternatively, should we move it back to the default $env:AppData\jjui and then remove persist?
• I am not sure which is a better solution.