.github/workflows: only run bench all on packages with benchmarks

Drops time by several minutes.

Also, on top of that: skip building variant CLIs on the race builder
(29s), and getting qemu (15s).

Updates #9182

Change-Id: I979e02ab8c0daeebf5200459c9e4458a1f62f728
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f