util/syspolicy: finish adding ts_omit_syspolicy build tags, tests

Fixes #16998
Updates #12614

Change-Id: Idf2b1657898111df4be31f356091b2376d0d7f0b
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