MVTSettings.initialise() constructs the settings once with load_env=False
and writes the result to config.yaml, so that values taken from MVT_*
environment variables are never persisted, then constructs them again
with the environment applied.
Since #716 settings_customise_sources() has added env_settings
unconditionally, making load_env dead code. Any MVT_* variable set for a
single run, including MVT_IOS_BACKUP_PASSWORD, MVT_ANDROID_BACKUP_PASSWORD
and MVT_VT_API_KEY, was written in plaintext to config.yaml and read back
on every later run.
Only add env_settings when load_env is true, and add a regression test.
Fixes#915
