Fix required tzdata package since Hugo v0.87

Since Hugo v0.87, sites with a configured timeZone config param will fail at build time in systems without a time zone IANA database.

Fail on invalid time zone 4d221ce4 @Bep #8832

Source: https://gohugo.io/news/0.87.0-relnotes/#other

The list of valid values may be system dependent

Source: https://gohugo.io/getting-started/configuration/#timezone

Test case

My site was failing at d9a6ef8e, but works after adding the tzdata package in GitLab CI's before_script hook at 7147bf94.