Load timezone database from the OS

[dcastro]

Clarification and motivation

At the moment, we're using Data.Time.TZInfo.fromLabel to fetch the timezone rules for some given timezone.

tzbot/src/TzBot/TimeReference.hs

Lines 140 to 146 in 4c2315e

	eithTzInfo :: Either UnknownTimeZoneAbbrev TZI.TZInfo
	eithTzInfo = case timeRef.trLocationRef of
	Nothing -> pure $ TZI.fromLabel sendersTZLabel
	Just (TimeZoneRef tzLabel) -> pure $ TZI.fromLabel tzLabel
	Just (OffsetRef offset) -> pure $ tzInfoFromOffset offset
	Just (TimeZoneAbbreviationRef abbrev) -> pure $ tzInfoFromOffset $ abbrev.tzaiOffsetMinutes
	Just (UnknownTimeZoneAbbreviationRef unknownAbbrev) -> Left unknownAbbrev

However, as the haddocks state, this uses the IANA database embedded inside the tzdata package:

Timezone rules are constantly getting updated and, unless we constantly upgrade the tzdata dependency to the latest version, eventually we'll be using old timezone rules, leading to wrong conversions.

Instead, we should use the operating system's timezone database, using Data.Time.TZInfo.loadFromSystem.

Note:

• Let's first check with the SRE team whether the server indeed contains this database, and that it is updated regularly.
• loadFromSystem will not work on Windows. We should document that the server cannot run on Windows.
  • (we could workaround this by defaulting to using fromLabel on windows, but if there's no need to support windows, then it's not worth doing)