cmd/go: update PWD variable for 'go generate'

Most subprocess invocations in the go command use base.AppendPWD to
append an accurate value of PWD to the command's environment, which can
speed up calls like os.Getwd and also help to provide less-confusing
output from scripts. Update `go generate` to do so.

Fixes #43862

Change-Id: I3b756f1532b2d922f7d74fd86414d5567a0122c0

src/cmd/go/testdata/script/generate.txt

@@ -28,7 +28,7 @@ stdout 'main_test'
 
 # Test go generate provides the right "$PWD"
 go generate './generate/env_pwd.go'
-stdout $WORK'/gopath/src/generate'
+stdout $WORK'[/\\]gopath[/\\]src[/\\]generate'
 
 -- echo.go --
 package main