nixos-unstable log file unwriteable

```
Jun 15 01:25:52 dell-nixos systemd[1]: Starting Nginx Web Server...
-- Subject: Unit nginx.service has begun start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit nginx.service has begun starting up.
Jun 15 01:25:52 dell-nixos nginx-pre-start[19878]: nginx: [warn] could not build optimal types_hash, you should increase either types_hash_max_size: 1024 or types_hash_bucket_size: 64; ignoring types_hash_bucket_size
Jun 15 01:25:52 dell-nixos nginx-pre-start[19878]: nginx: the configuration file /nix/store/6flkwwcrd2smyxq9s88jncjqsik55z76-nginx.conf syntax is ok
Jun 15 01:25:52 dell-nixos nginx-pre-start[19878]: nginx: [emerg] open() "/nix/store/sg297sr4iji76br0511z17k4x35mpcbl-nginx-1.18.0/logs/access.log" failed (2: No such file or directory)
Jun 15 01:25:52 dell-nixos nginx-pre-start[19878]: nginx: configuration file /nix/store/6flkwwcrd2smyxq9s88jncjqsik55z76-nginx.conf test failed
Jun 15 01:25:52 dell-nixos systemd[1]: nginx.service: Control process exited, code=exited, status=1/FAILURE
Jun 15 01:25:52 dell-nixos systemd[1]: nginx.service: Failed with result 'exit-code'.
Jun 15 01:25:52 dell-nixos systemd[1]: Failed to start Nginx Web Server.
-- Subject: Unit nginx.service has failed
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit nginx.service has failed.
--
-- The result is RESULT.
```
https://github.com/nh2/nix-binary-cache-proxy/blob/b144bad7e95fc78ab50b2230df4920938899dab0/nginx-binary-cache-proxy.nix#L57
it used to write to `/var/spool/nginx/logs/access.log`
but now tries to write to the read only store. `/nix/store/sg297sr4iji76br0511z17k4x35mpcbl-nginx-1.18.0/logs/access.log`

seems likely to be related to this https://github.com/NixOS/nixpkgs/pull/85862#issuecomment-619948672