Revert "[tailscale] cmd/dist: always default to CGO_ENABLED="""

This reverts commit a4256bc.

This now breaks x/tools/go/packages (and thus depaware) for some reason, but
this also doesn't seem necessary anymore--- the Go 1.24.0 binaries (cmd/go)
are already statically linked.

So I'm going to revert this and run it through our CI and verify this
is truly not needed anymore. I haven't dug into when/why Go changed
its behavior yet.

Updates #52

Author: bradfitz

src/cmd/dist/buildgo.go

@@ -7,6 +7,7 @@ package main
 import (
 	"fmt"
 	"io"
+	"os"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -118,7 +119,7 @@ func mkzcgo(dir, file string) {
 	writeHeader(&buf)
 	fmt.Fprintf(&buf, "package build\n")
 	fmt.Fprintln(&buf)
-	fmt.Fprintf(&buf, "const defaultCGO_ENABLED = %q\n", "")
+	fmt.Fprintf(&buf, "const defaultCGO_ENABLED = %s\n", quote(os.Getenv("CGO_ENABLED")))
 
 	writefile(buf.String(), file, writeSkipSame)
 }