cmd/go: set up git identity for build_buildvcs_auto.txt

Just like in other tests like get_dotfiles.txt or
version_buildvcs_git.txt. Without it, I get a failure on my machine:

	fatal: empty ident name (for <[email protected]>) not allowed

Change-Id: I1c17c0d58c539b59154570b5438c7bd850bac5aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/416095
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Daniel Martí <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: mvdan

Diff for: src/cmd/go/testdata/script/build_buildvcs_auto.txt

@@ -6,11 +6,15 @@
 
 cd sub
 exec git init .
+exec git config user.name 'Nameless Gopher'
+exec git config user.email '[email protected]'
 exec git add sub.go
 exec git commit -m 'initial state'
 cd ..
 
 exec git init
+exec git config user.name 'Nameless Gopher'
+exec git config user.email '[email protected]'
 exec git submodule add ./sub
 exec git add go.mod example.go
 exec git commit -m 'initial state'