Add a --test-version flag to run the natlab VM tests against
released tailscale/tailscaled binaries downloaded from
pkgs.tailscale.com instead of building from the source tree.
The value can be a concrete release like "1.97.255", or "stable" /
"unstable" which resolve to the latest TarballsVersion on that track
via pkgs.tailscale.com/<track>/?mode=json. The track for a concrete
version is derived from its minor (even=stable, odd=unstable). The
host architecture (amd64 or arm64) selects the tarball.
Tarballs are cached + extracted under
~/.cache/tailscale-vmtest/builds/<version>_<arch>/ so they are not
re-fetched per test. tta is still always built from the local tree.
Cloud VMs (Ubuntu, Debian) pick up the downloaded binaries via the
existing files.tailscale file server. Non-Linux GOOS (FreeBSD) falls
back to building from source since pkgs.tailscale.com only ships
Linux tarballs. Gokrazy nodes continue to use binaries baked into
the gokrazy image; --test-version is a no-op for them.
Updates #13038
Change-Id: I213ef7db362dd17bf69d2685cbf2ab0ec5a3fee1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
