Improve validation for `version` input

[Zebradil]

The action fails on downloading checksum:

Run tailscale/github-action@306e68a486fd2350f2bfc3b19fcd143891a4a2d8
Resolved Tailscale version: v1.98.10
▶️ curl https://pkgs.tailscale.com/stable/tailscale_v1.98.10_amd64.tgz.sha256
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
Error: curl failed with exit code 22
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0    19    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404

Even though the corresponding archive is listed on https://pkgs.tailscale.com/stable/#static: https://pkgs.tailscale.com/stable/tailscale_1.98.10_amd64.tgz - and it is downloadable.

[Zebradil]

Solved the issue by using 1.98.10 instead of v1.98.10 (notice the v prefix).

So, not a bug, but worth adding some validation (Resolved Tailscale version: v1.98.10 log line makes impression that the version is totally fine).
I leave this opened for maintainers to decide, feel free to close.

[neinkeinkaffee]

Thanks for raising this @Zebradil! The version input needs some extra validation.