grafana: explicitly declare subPackages

The build itself is pretty quick now:

    buildPhase completed in 2 minutes 46 seconds

in contrast to

    buildPhase completed in 5 minutes 22 seconds

before on the same machine (Hetzner AX41 NVMe with
AMD Ryzen 5 3600 6-Core Processor and 64GiB RAM).

Downside of that is that no tests are now executed, but I'm inclined to
make that sacrifice for now. Especially considering that a fix for that
is on the horizon with NixOS#284568.

(cherry picked from commit f220ea4)

pkgs/servers/monitoring/grafana/default.nix

@@ -34,7 +34,7 @@ buildGoModule rec {
  pname = "grafana";
  version = "10.3.3";
 
- excludedPackages = [ "alert_webhook_listener" "clean-swagger" "release_publisher" "slow_proxy" "slow_proxy_mac" "macaron" "devenv" "modowners" ];
+ subPackages = [ "pkg/cmd/grafana" "pkg/cmd/grafana-server" "pkg/cmd/grafana-cli" ];
 
  src = fetchFromGitHub {
  owner = "grafana";