Add timezone validation

[mazzz1y]

My machine accidentally has a symlink in /etc/timezone instead of a proper timezone file, and I spent some time investigating why cloud-init doesn't install anything in the guest, with this error:

[    5.994555] cloud-init[685]: 2025-07-03 09:19:09,880 - util.py[WARNING]: Failed loading yaml blob. unacceptable character #x0000: special characters are not allowed
[    5.996367] cloud-init[685]:   in "<unicode string>", position 88
[    5.997206] cloud-init[685]: 2025-07-03 09:19:09,880 - cloud_config.py[WARNING]: Failed at merging in cloud config part from part-001: empty cloud config

This PR adds timezone validation using the native Go way. In my case, it prints this warning and falls back to an empty value:

WARN[0000] invalid timezone "TZif2\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\b\x00\x00..." 

[alexandear]

Please add unit tests to cover your changes

[mazzz1y]

I've added platform-specific build constraints as @AkihiroSuda suggested, refactored the function, and added tests for the time-zone extraction function.

[AkihiroSuda]

Thanks