unix: use Go 1.21+ clear built-in

Inspired by CL 698495.

Change-Id: Ic6ff68a60d41b6ffb6a28f523597c5d36c1e4d2f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/704915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Mark Freeman <markfreeman@google.com>