demo: fix socket location for bazel sandbox

[benbardin]

This resolves a failure when demo mode tried to start a socket in $HOME without permissions.

[cockroach-teamcity]

This change is 

[RaduBerinde]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @rickystewart)

---

testserver/testservernode.go line 145 at r1 (raw file):

	if ts.serverArgs.demoMode {
		// Prefer a very short path to stay under Unix-domain socket length limits.
		preferred := filepath.Join("/tmp", fmt.Sprintf("cd-%d", os.Getpid()))

[nit] We should use os.MkdirTemp

---

testserver/testservernode.go line 148 at r1 (raw file):

		demoHome := preferred
		if err := os.MkdirAll(demoHome, 0700); err != nil {
			log.Printf("%s: /tmp not writable for demo HOME (%v); falling back to base dir %s", testserverMessagePrefix, err, ts.baseDir)

Is this necessary (did you see this error under bazel)?

[benbardin]

Switched to os.MkdirTemp and removed the fallback, as you're right - it is not necessary. Thanks!