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
GitHub-Last-Rev: 3ec8da2
GitHub-Pull-Request: #43940
Reviewed-on: https://go-review.googlesource.com/c/go/+/287152
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Baokun Lee <bk@golangcn.org>

src/cmd/go/internal/generate/generate.go

@@ -334,6 +334,7 @@ func (g *Generator) setEnv() {
 		"GOPACKAGE=" + g.pkg,
 		"DOLLAR=" + "$",
 	}
+	g.env = base.AppendPWD(g.env, g.dir)
 }
 
 // split breaks the line into words, evaluating quoted

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

@@ -26,6 +26,10 @@ stdout 'yes' # flag.go should select yes
 go generate './generate/env_test.go'
 stdout 'main_test'
 
+# Test go generate provides the right "$PWD"
+go generate './generate/env_pwd.go'
+stdout $WORK'[/\\]gopath[/\\]src[/\\]generate'
+
 -- echo.go --
 package main
 
@@ -88,4 +92,8 @@ package p
 -- generate/env_test.go --
 package main_test
 
-//go:generate echo $GOPACKAGE
+//go:generate echo $GOPACKAGE
+-- generate/env_pwd.go --
+package p
+
+//go:generate echo $PWD