tool/gocross: merge user's build tags and implicitly added build tags together

Fixes tailscale/corp#15058

Change-Id: I7e539b3324153077597f30385a2cb540846e8bdc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f