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.
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