Use `west sdk`

[beriberikix]

zephyrproject-rtos/zephyr#76283 adds a west command that does the fetching of the SDK. It isn't much different than downloading the shell script and running it but it's a little bit cleaner.

[henrikbrixandersen]

Using west sdk would impose a minimum supported Zephyr version.

[fabiobaltieri]

Using west sdk would impose a minimum supported Zephyr version.

Good point, and it's not in the LTS so that's a bit of a problem. May be a change big enough to justify a v2 tag. Either way, I guess we have to wait for at least 4.0 before it make sense to look into it.