cibuild.On() returns true for any CI environment that sets CI=true,
including Alpine Linux's package build CI. TestTsgoRevInCacheKey was
guarded by cibuild.On() (or use of tsgo), so it ran under Alpine's CI
with stock Go, where go.toolchain.rev isn't blended into build cache
keys, and unsurprisingly failed.
Add cibuild.OnTailscaleCI, which keys off GITHUB_REPOSITORY_OWNER to
distinguish tailscale/tailscale's own GitHub Actions CI from arbitrary
downstream CI, and use it in TestTsgoRevInCacheKey.
Fixes#19754
Change-Id: Id31cfe71903a235f1460dca1e2fdf334e3ba1ee5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
ef1bb5ac16