Better: Add netbsd support.

[ylecuyer]

Before:

netbsd$ cat .tool-versions 
golang 1.23.11
netbsd$ asdf install
Platform 'netbsd' not supported!
failed to run download callback: exit status 1

After:

netbsd$ cat .tool-versions 
golang 1.23.11
netbsd$ asdf install
Platform 'netbsd' supported!
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 69.8M  100 69.8M    0     0  17.2M      0  0:00:04  0:00:04 --:--:-- 17.2M
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    64  100    64    0     0    821      0 --:--:-- --:--:-- --:--:--   831
verifying checksum
checksum verified
netbsd$ asdf list golang
 *1.23.11
netbsd$ go version
go version go1.23.11 netbsd/amd64