cmd/go: use AppendPath(OrigEnv) as env for tool executable

Before this change, when go tool wass used to start a tool defined in a
go.mod tool directive, it used the environment the go command was
running in. The issue with doing that is that the go command sets
various environment variables from the computed environment when
invoking a subcommand. That is used to standardise the environment for
the various tools invoked by the go command, but it is not the
expectatation of tools invoked by the go command, especially since those
environment variables may change the behavior of the tool run. Instead
use the same environment we use in go run to start the executable: the
original environment (with minor modifications) saved before we start
explicitly setting the envornment, with GOROOT/bin added to the path so
that sub commands that run the go tool use the proper go tool binary.

Fixes #70544

Change-Id: Ifbf0040a2543113638eec7232323eb9de1d61529
Reviewed-on: https://go-review.googlesource.com/c/go/+/631836
Reviewed-by: Conrad Irwin <conrad.irwin@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/tool/tool.go

@@ -309,12 +309,18 @@ func runBuiltTool(b *work.Builder, ctx context.Context, a *work.Action) error {
 		return nil
 	}
 
+	// Use same environment go run uses to start the executable:
+	// the original environment with cfg.GOROOTbin added to the path.
+	env := slices.Clip(cfg.OrigEnv)
+	env = base.AppendPATH(env)
+
 	toolCmd := &exec.Cmd{
 		Path:   cmdline[0],
 		Args:   cmdline,
 		Stdin:  os.Stdin,
 		Stdout: os.Stdout,
 		Stderr: os.Stderr,
+		Env:    env,
 	}
 	err := toolCmd.Start()
 	if err == nil {