nixos/kimai: fix upgrading to Kimai 2.32 by clearing cache earlier
During testing of Kimai 2.32, it's been found that fixes done in PR
371917 [^1] is not enough to prevent error in the case of upgrading from
2.31 to 2.32.
Hinted by an upstream issue [^2], make sure that the cache is cleared
before running `kimai:install`, not after. This fixes upgrading from
2.31 to 2.32, and should prevent similar issue from popping up again.
[^1]: https://github.com/NixOS/nixpkgs/pull/371917
[^2]: https://github.com/kimai/kimai/issues/5437