Default time zone database assumes that `/etc/localtime` is present

Prerequisites

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
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Description

We currently have no fallback if /etc/localtime is not present on a system. We should at least look at the TZ environment variable before failing.

Impact

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