archive: fix order of struct items to reduce memory usage

Found by fieldalignment:
        archive.go:46:14: struct with 88 pointer bytes could be 56
        archive.go:66:18: struct with 24 pointer bytes could be 16

The first one probably doesn't matter that much, there's only like 2 of
those objects that are instantiated at runtime. However, there are many
ArchiveItems (hundreds or more depending on the archives compositions)

internal/archive/archive.go

@@ -44,11 +44,11 @@ const (
 )
 
 type Archive struct {
-	items           archiveItems
 	cpioWriter      *cpio.Writer
 	buf             *bytes.Buffer
 	compress_format CompressFormat
 	compress_level  CompressLevel
+	items           archiveItems
 }
 
 func New(format CompressFormat, level CompressLevel) (*Archive, error) {
@@ -64,8 +64,8 @@ func New(format CompressFormat, level CompressLevel) (*Archive, error) {
 }
 
 type archiveItem struct {
-	sourcePath string
 	header     *cpio.Header
+	sourcePath string
 }
 
 type archiveItems struct {