Persist variable IDF_GITHUB_ASSETS

[georgik]

The installers sets variable IDF_GITHUB_ASSETS to value dl.espressif.com/github_assets during installation process when user checks the option to prefer Github mirror (e.g. in case when Github is not accessible from the location).
The value is transient and when user starts new shell with IDF the value is not set.
Persist the value in environment variables.
Add also option to remove the variable when user unchecks the value in the installer.