Sort files in cpio archive

As discussed on Matrix, in order to produce a reproducible archive that can be compared with e.g. diffoscope it would be great if the list of files in the archive would be sorted deterministically.

A WIP patch that actually doesn't work can be found here:

diff --git a/pkgs/archive/archive.go b/pkgs/archive/archive.go
index c47c2e2..d7a6caf 100644
--- a/pkgs/archive/archive.go
+++ b/pkgs/archive/archive.go
@@ -13,6 +13,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"sort"
 	"strings"
 )
 
@@ -181,8 +182,16 @@ func (archive *Archive) writeCpio() error {
 		archive.addDir(dir)
 	}
 
+	// Sort files, so they have a deterministic order in the archive
+	keys := make([]string, 0, len(archive.Files))
+	for k := range archive.Files {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+
 	// Write files and any missing parent dirs
-	for file, imported := range archive.Files {
+	for _, file := range keys {
+		imported := archive.Files[file]
 		if imported {
 			continue
 		}

I assume that also in func (archive *Archive) writeCpio() and func (archive *Archive) addDir(dir string) this sorting would need to be done.