fails on itself at tip

[jba]

unparam fails at tip.
This shows it failing on itself. I've also seen it fail on a package that imports "context" with a similar message.

> gotip download                                                                                                      
Updating the go development tree...                                                                                   
remote: Total 1085 (delta 602), reused 993 (delta 602)                                                                
Receiving objects: 100% (1085/1085), 1.79 MiB | 9.23 MiB/s, done.                                                     
Resolving deltas: 100% (602/602), completed with 133 local objects.                                                   
From https://go.googlesource.com/go                                                                                   
 * branch            master     -> FETCH_HEAD                                                                         
   0fa53e4..474ebb9  master     -> origin/master                                                                      
Previous HEAD position was 0fa53e4 spec: fix link for instantiations                                                  
HEAD is now at 474ebb9 syscall: avoid writing to p when Pipe(p) fails  
...
> git clone https://github.com/mvdan/unparam 
> cd unparam
> gotip run .                                                                                                         
2021/12/09 09:32:12 internal error: package "flag" without types was imported from "mvdan.cc/unparam"                 
exit status 1

[mvdan]

I think this is golang/go#49164 (comment); should be an easy fix to upgrade the x/tools dep.