fix: Protect codersdk.Client SessionToken so it can be updated (#4965)

This feature is used by the coder agent to exchange a new token. By
protecting the SessionToken via mutex we ensure there are no data races
when accessing it.

coderd/gitsshkey_test.go

@@ -134,7 +134,7 @@ func TestAgentGitSSHKey(t *testing.T) {
 	coderdtest.AwaitWorkspaceBuildJob(t, client, workspace.LatestBuild.ID)
 
 	agentClient := codersdk.New(client.URL)
-	agentClient.SessionToken = authToken
+	agentClient.SetSessionToken(authToken)
 
 	ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
 	defer cancel()