nixos/grafana-agent: use lib.getExe as binary names changed between updates

and `lib.getExe` allows a safe handling and potential backport of this.
But for that to work it would require 22.11 to set `pkgs.grafana-agent.meta.mainProgram = "agent"`

Relevant upstream release: https://github.com/grafana/agent/releases/tag/v0.31.0

nixos/modules/services/monitoring/grafana-agent.nix

@@ -140,7 +140,7 @@ in
         # We can't use Environment=HOSTNAME=%H, as it doesn't include the domain part.
         export HOSTNAME=$(< /proc/sys/kernel/hostname)
 
-        exec ${cfg.package}/bin/agent -config.expand-env -config.file ${configFile}
+        exec ${lib.getExe cfg.package} -config.expand-env -config.file ${configFile}
       '';
       serviceConfig = {
         Restart = "always";