feat: Use environment variables and startup script in agent (#1147)

These values were ignored. Environment variables are applied to
new sessions, and are refreshed on reconnect. This is cool because
a workspace could be updated with new environment variables without
requiring a complete start/stop.

The startup script is only ran once regardless of changes, which
feels like the expected behavior.

coderd/workspaceagents_test.go

@@ -102,6 +102,8 @@ func TestWorkspaceAgentListen(t *testing.T) {
 	})
 	_, err = conn.Ping()
 	require.NoError(t, err)
+	_, err = agentClient.WorkspaceAgent(context.Background(), codersdk.Me)
+	require.NoError(t, err)
 }
 
 func TestWorkspaceAgentTURN(t *testing.T) {