plugin: install tools with -mod=mod

Install tools with -mod=mod instead of -mod=readonly to avoid problem
when some dependency of the tool being updated needs a newer version of
Go than some other dependency while also still satisfying the original
motivation for using the -mod flag (i.e. installing tools even when
GOFLAGSS=-mod=vendor).

Fixes fatih#3446

plugin/go.vim

@@ -107,7 +107,7 @@ function! s:GoInstallBinaries(updateBinaries, ...)
     set noshellslash
   endif
 
-  let l:get_base_cmd = ['go', 'install', '-v', '-mod=readonly']
+  let l:get_base_cmd = ['go', 'install', '-v', '-mod=mod']
 
   " Filter packages from arguments (if any).
   let l:packages = {}