feature/featuretags: skip TestAllOmitBuildTagsDeclared when not in a git repo

This test was failing on Alpine's CI which had 'git' but wasn't in a git repo:

https://github.com/tailscale/tailscale/commit/036b6a12621306da8368b167deb9858d4a8d6ce9#commitcomment-180001647

Updates #12614

Change-Id: Ic1b8856aaf020788a2a57e48738851e13ea85a93
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f