[core] Do not start the plugin loader on non-clients (#16111)

The plugin loader loads task and device driver plugins which are not
used on server nodes.

command/agent/plugins_test.go

@@ -0,0 +1,33 @@
+package agent
+
+import (
+	"testing"
+
+	"github.com/hashicorp/nomad/api"
+	"github.com/shoenig/test/must"
+)
+
+func TestPlugins_WhenNotClientSkip(t *testing.T) {
+	s, _, _ := testServer(t, false, nil)
+	must.Nil(t, s.Agent.pluginSingletonLoader)
+}
+
+func TestPlugins_WhenClientRun(t *testing.T) {
+	s, _, _ := testServer(t, true, nil)
+	must.NotNil(t, s.Agent.pluginSingletonLoader)
+}
+
+func testServer(t *testing.T, runClient bool, cb func(*Config)) (*TestAgent, *api.Client, string) {
+	// Make a new test server
+	a := NewTestAgent(t, t.Name(), func(config *Config) {
+		config.Client.Enabled = runClient
+
+		if cb != nil {
+			cb(config)
+		}
+	})
+	t.Cleanup(a.Shutdown)
+
+	c := a.Client()
+	return a, c, a.HTTPAddr()
+}