The kernel version parser used strings.Cut with "-" to handle versions
like "5.4.0-76-generic", but Debian uses "+" in versions like
"6.12.41+deb13-amd64".
Use strings.IndexAny to find the first "-" or "+" and truncate there.
Fixes TestKernelVersion on Debian systems.
Fixes#19395
Change-Id: I70e5f95682d54baf908e51f9f4b51c130b00aaaa
Co-Authored-By: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>
