This commit adds the user-facing version to the tailscale version
output.
Prior to this change, it used a hardcoded fallback that is fairly
infrequently updated (https://github.com/tailscale/tailscale/commits/main/version/version.go).
After this change, we print the user-friendlier tag version, which is
helpful when wanting to e.g check to see if you have a version that is
compatible with a feature like [magic dns][magic-dns].
E.g:
```
[nixpkgs(dani/tailscale-version)] $ ./result/bin/tailscale version
1.0.5
```
[magic-dns]: https://tailscale.com/kb/1081/magic-dns
7526ae55d7