thermald: also install thermal-conf.xml into $out

Otherwise you get errors like this when running `thermald.service` from
the `services.thermald` module:

```
[WARN]22 CPUID levels; family:model:stepping 0x6:8e:a (6:142:10)
[WARN]Polling mode is enabled: 4
[WARN]sensor id 10 : No temp sysfs for reading raw temp
I/O warning : failed to load external entity "/nix/store/7d7cfc1949g7n7ywx47a0dsfz3b3rix5-thermald-1.9.1/etc/thermald/thermal-conf.xml"
[WARN]error: could not parse file /nix/store/7d7cfc1949g7n7ywx47a0dsfz3b3rix5-thermald-1.9.1/etc/thermald/thermal-conf.xml
[WARN]sysfs open failed
I/O warning : failed to load external entity "/nix/store/7d7cfc1949g7n7ywx47a0dsfz3b3rix5-thermald-1.9.1/etc/thermald/thermal-conf.xml"
[WARN]error: could not parse file /nix/store/7d7cfc1949g7n7ywx47a0dsfz3b3rix5-thermald-1.9.1/etc/thermald/thermal-conf.xml
I/O warning : failed to load external entity "/nix/store/7d7cfc1949g7n7ywx47a0dsfz3b3rix5-thermald-1.9.1/etc/thermald/thermal-conf.xml"
[WARN]error: could not parse file /nix/store/7d7cfc1949g7n7ywx47a0dsfz3b3rix5-thermald-1.9.1/etc/thermald/thermal-conf.xml
```