scd4x: data race in its unit test

[maruel]

Describe the bug
A race condition in the scd4x driver.

To Reproduce

go test -race, e.g. Example: https://github.com/periph/devices/actions/runs/12216048416/job/34078745883?pr=8

==================
WARNING: DATA RACE
Read at 0x00c000218078 by goroutine 18:
  periph.io/x/devices/v3/scd4x.(*Dev).Sense()
      D:/a/devices/devices/scd4x/scd4x.go:533 +0xd9
  periph.io/x/devices/v3/scd4x.(*Dev).SenseContinuous.func1()
      D:/a/devices/devices/scd4x/scd4x.go:600 +0x351

Previous write at 0x00c000218078 by goroutine 19:
  periph.io/x/devices/v3/scd4x.(*Dev).Halt()
      D:/a/devices/devices/scd4x/scd4x.go:373 +0x[133](https://github.com/periph/devices/actions/runs/12216048416/job/34078745883?pr=83#step:9:134)
  periph.io/x/devices/v3/scd4x.TestSenseContinuous.func2()
      D:/a/devices/devices/scd4x/scd4x_test.go:257 +0x4f

Goroutine 18 (running) created at:
  periph.io/x/devices/v3/scd4x.(*Dev).SenseContinuous()
      D:/a/devices/devices/scd4x/scd4x.go:583 +0x1bb
  periph.io/x/devices/v3/scd4x.TestSenseContinuous()
      D:/a/devices/devices/scd4x/scd4x_test.go:250 +0x33c
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.22.6/x64/src/testing/testing.go:1689 +0x21e
  testing.(*T).Run.gowrap1()
      C:/hostedtoolcache/windows/go/1.22.6/x64/src/testing/testing.go:1742 +0x44

Goroutine 19 (running) created at:
  periph.io/x/devices/v3/scd4x.TestSenseContinuous()
      D:/a/devices/devices/scd4x/scd4x_test.go:255 +0x479
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.22.6/x64/src/testing/testing.go:1689 +0x21e
  testing.(*T).Run.gowrap1()
      C:/hostedtoolcache/windows/go/1.22.6/x64/src/testing/testing.go:1742 +0x44
==================

https://github.com/periph/devices/blob/main/scd4x/scd4x.go#L533
https://github.com/periph/devices/blob/main/scd4x/scd4x.go#L373

Fix

Add a d.mu.Lock() + Unlock around line 533 when reading d.sensing.