time: Fix toml zoneinfo inconsistency

[satotake]

TOML's time representation has known issues about zoneinfo name such as PDT in hugo

• https://github.com/gohugoio/hugo/pull/9082
• https://github.com/gohugoio/hugo/pull/9982
• https://github.com/gohugoio/hugo/issues/8895
• https://github.com/gohugoio/hugo/issues/9996

These are caused because

• TOML's time format has only offset
  • zoneinfo cannot be retrieved with offset value
• TZ environment variable can change the behaviour
• cast.ToTimeInDefaultLocationE may or may not use location arg

Given that hugo often is used on both local and remote machine, and as #9996 said hugo supports multilingual site generation,

• TZ envvar should not have any effect on this
  • Remote environments can be hard to debug
• If only timeZone is set explicitly in config file and offsets of time are matched with it, hugo should show zoneinfo
  • Otherwise, zoneinfo should not be displayed
• hugo should handle TOML's time object and time string equally

To meet these,

• assure to set location derived from conf to time of TOML if possible
• add tests

Close https://github.com/gohugoio/hugo/issues/9996