rebased the schema override patch, adding the same upstream safety but
not duplicating the ability to pass multiple directories as this does
not appear to be necessary for nix overrides
For some reason, the GNOME 3.26 update broke the overrides. It turns
out the overrides now need to come before the overriden schemas in the
XDG_DATA_DIRS variable. This is not possible in general due to applications
prefixing the variable (e.g. in wrapGAppsHook).
To fix this, a new environment variable NIX_GSETTINGS_OVERRIDES_DIR
was introduced. It has greater priority than XDG_DATA_DIRS but lower
than GSETTINGS_SCHEMA_DIR. A separate variable was chosen in order not
to block the built-in one for users.
