Configuration values can be loaded from the system environment by adding
a "TUTOR_" prefix.
Environment values supersede values from the user configuration file, so
that we can set values from the command line with "KEY=VAL tutor config
save --silent" even when KEY is already present in the user
configuration file.
Environment is no longer generated separately for each target, but only
once the configuration is saved.
Note that the environment is automatically updated during
re-configuration, based on a "version" file stored in the environment.
