util/cibuild, cache_key_test: skip TestTsgoRevInCacheKey outside Tailscale CI

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>

cache_key_test.go

@@ -21,8 +21,8 @@ import (
 func TestTsgoRevInCacheKey(t *testing.T) {
 	goRoot := goEnv(t, "GOROOT")
 	isTsgo := strings.Contains(goRoot, "/.cache/tsgo/")
-	if !cibuild.On() && !isTsgo {
-		t.Skip("skipping; not in CI and not using the Tailscale Go toolchain")
+	if !cibuild.OnTailscaleCI() && !isTsgo {
+		t.Skip("skipping; not in Tailscale CI and not using the Tailscale Go toolchain")
 	}
 
 	rev := strings.TrimSpace(GoToolchainRev)