since 2.8.5, shairport-sync uses the directory path sysconfdir to
determine where to place the configuration file shairport-sync.conf.
This led to shairport-sync complaining at startup:
> configuration file name "/nix/store/i8gb0hsb2zczdl8g1h73pbcb952585l9-shairport-sync-3.2.2/etc/shairport-sync.conf" can not be resolved.
Looking for a configuration file there doesn't make much sense, as we'd
have to include configuration in the shairport-sync derivation itself.
By setting sysconfdir to /etc/, shairport-sync will try to read its
configuration from /etc/shairport-sync.conf, which is a more sane
default.
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