cmd/go: use builtin version and reexec for env

Skip execing for version (most reliable) and attempt to reexec
for env, falling back to the go in GOROOT

Fixes #43981

Change-Id: I17fb84d36036807274eecca3d4f64b3add9b9483
Reviewed-on: https://go-review.googlesource.com/c/go/+/288693
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/bug/bug.go

@@ -81,7 +81,7 @@ func printGoVersion(w io.Writer) {
 	fmt.Fprintf(w, "### What version of Go are you using (`go version`)?\n\n")
 	fmt.Fprintf(w, "<pre>\n")
 	fmt.Fprintf(w, "$ go version\n")
-	printCmdOut(w, "", "go", "version")
+	fmt.Fprintf(w, "go version %s %s/%s\n", runtime.Version(), runtime.GOOS, runtime.GOARCH)
 	fmt.Fprintf(w, "</pre>\n")
 	fmt.Fprintf(w, "\n")
 }
@@ -90,7 +90,11 @@ func printEnvDetails(w io.Writer) {
 	fmt.Fprintf(w, "### What operating system and processor architecture are you using (`go env`)?\n\n")
 	fmt.Fprintf(w, "<details><summary><code>go env</code> Output</summary><br><pre>\n")
 	fmt.Fprintf(w, "$ go env\n")
-	printCmdOut(w, "", "go", "env")
+	goexe, err := os.Executable()
+	if err != nil {
+		goexe = filepath.Join(runtime.GOROOT(), "bin/go")
+	}
+	printCmdOut(w, "", goexe, "env")
 	printGoDetails(w)
 	printOSDetails(w)
 	printCDetails(w)

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

@@ -1,9 +1,11 @@
 # Verify that go bug creates the appropriate URL issue body
 
 [!linux] skip
+[short] skip
 
 go install
-env BROWSER=$GOPATH/bin/browser
+go build -o $TMPDIR/go ./go
+env BROWSER=$GOPATH/bin/browser PATH=$TMPDIR:$PATH
 go bug
 exists $TMPDIR/browser
 grep '^go version' $TMPDIR/browser
@@ -44,3 +46,13 @@ func main() {
 	}
 }
 
+-- go/main.go --
+package main
+
+import (
+    "os"
+)
+
+func main() {
+    os.Exit(1)
+}