std.Io.HostName: fix a bug where expand would include the terminator

[notcancername]

the zero terminator was included in the output of
HostName.expand, which would try and fail to HostName.validate it
because it contained a zero byte

closes #25811

[notcancername]

CI failure seems spurious:

Main test suite...
test
+- test-incremental
   +- incr-check 'add_decl_namespaced' failure
warning: failed to delete tree 'tmp_7beb1e3fe3d5fb1d': FileBusy
error: expected compile errors but compilation incorrectly succeeded
error: process exited with code 1 (expected exited with code 0)

[squeek502]

Thanks for the fix, apologies for not seeing this sooner.

I'm going to take #25909 instead just because the test data is a bit nicer, and because anything depending on the NUL byte being written is a bug that should be found and fixed, as the sentinel is not encoded in the returned type.