envknob: add time.Duration knob support

Updates #6818

Change-Id: I9c8147c02fb514f9f6f1f272bdb0f974c8b3ccbb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f