use nix-fast-build niks3 integration for cache pushing

we're getting a lot of errors like this:
```
post-build-hook: niks3-hook send: server error: enqueueing paths: inserting path "/nix/store/8kbld33jrw1izhja6yls35qxswndxiz4-unit-dbus.service": database is locked (5) (SQLITE_BUSY)
```
maybe the niks3 post build hook isn't really right for the parallelized builds we do, we should probably use the [nix-fast-build integration](https://github.com/Mic92/nix-fast-build#niks3-support) instead