nixos/vault-agent: fix ExecStart generation when a vault package is set

Without this fix, if `config.services.vault.package` is set to e.g.
`vault-bin` (instead of the `vault` default), then `ExecStart` gets
improperly generated without the `agent` positional argument because
`vault-bin` did not match `vault`, and the command for the `consul`
flavor gets generated instead.

To fix this, we can look at the `flavor` argument instead of the package
name.

nixos/modules/services/security/vault-agent.nix

@@ -125,7 +125,7 @@ let
         Group = instance.group;
         RuntimeDirectory = flavour;
         ExecStart = "${lib.getExe instance.package} ${
-          lib.optionalString ((lib.getName instance.package) == "vault") "agent"
+          lib.optionalString (flavour == "vault-agent") "agent"
         } -config ${configFile}";
         ExecReload = "${pkgs.coreutils}/bin/kill -SIGHUP $MAINPID";
         KillSignal = "SIGINT";