`from_unixtime` does not return RFC3339

[hiltontj]

Problem

Described and discussed in influxdata/influxdb#26637

The docs for from_unixtime claim to convert a UNIX epoch integer into an RFC3339 timestamp:

Converts an integer to RFC3339 timestamp format (YYYY-MM-DDT00:00:00.000000000Z). Input is parsed as a Unix nanosecond timestamp and returns the corresponding RFC3339 timestamp.

They actually convert the integer into an Arrow Timestamp. That just happens to be formatted as an RFC3339 timestamp when rendered in the console, to JSON, etc.