cmd/go: place GOROOT/bin at the beginning of PATH in 'go run'

This causes programs that use 'go' as a subprocess to use the same go
command as the parent 'go run' command.

Fixes #68005

Change-Id: I937cef474bf038a925bb74fc73e5f377b03e27b7
GitHub-Last-Rev: 9986537cad
GitHub-Pull-Request: golang/go#68040
Reviewed-on: https://go-review.googlesource.com/c/go/+/593255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/base/base.go

@@ -14,6 +14,7 @@ import (
 	"os"
 	"os/exec"
 	"reflect"
+	"slices"
 	"strings"
 	"sync"
 
@@ -211,7 +212,9 @@ func RunStdin(cmdline []string) {
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
-	cmd.Env = cfg.OrigEnv
+	env := slices.Clip(cfg.OrigEnv)
+	env = AppendPATH(env)
+	cmd.Env = env
 	StartSigHandlers()
 	if err := cmd.Run(); err != nil {
 		Errorf("%v", err)

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

@@ -0,0 +1,42 @@
+# https://go.dev/issue/68005: 'go run' should run the program with its own GOROOT/bin
+# at the beginning of $PATH.
+
+[short] skip
+
+[!GOOS:plan9] env PATH=
+[GOOS:plan9] env path=
+go run .
+
+[!GOOS:plan9] env PATH=$WORK${/}bin
+[GOOS:plan9] env path=$WORK${/}bin
+go run .
+
+-- go.mod --
+module example
+
+go 1.19
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+func main() {
+	got, err := exec.LookPath("go")
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+
+	want := filepath.Join(os.Getenv("GOROOT"), "bin", "go" + os.Getenv("GOEXE"))
+	if got != want {
+		fmt.Printf(`exec.LookPath("go") = %q; want %q\n`, got, want)
+		os.Exit(1)
+	}
+}
+-- $WORK/bin/README.txt --
+This directory contains no executables.