sbom: move too-late os.Chdir call earlier

This fixes errors such as:

2024/09/11 08:41:50 Error: build directory
"/home/michael/gokrazy/router7/builddir/github.com/stapelberg/prober7/cmd/probe"
does not exist in
"/home/michael/gokrazy/router7/builddir/github.com/gokrazy/serial-busybox"

internal/packer/sbom.go

@@ -143,11 +143,7 @@ func GenerateSBOM(cfg *config.Struct) ([]byte, SBOMWithHash, error) {
 			}
 		}
 
-		files := append([]*FileInfo{}, extraFiles[pkg]...)
-		if len(files) == 0 {
-			continue
-		}
-
+		// Restore the working directory before possibly 'continue'ing.
 		if err := os.Chdir(config.InstancePath()); err != nil {
 			if os.IsNotExist(err) {
 				// best-effort compatibility for old setups
@@ -156,6 +152,11 @@ func GenerateSBOM(cfg *config.Struct) ([]byte, SBOMWithHash, error) {
 			}
 		}
 
+		files := append([]*FileInfo{}, extraFiles[pkg]...)
+		if len(files) == 0 {
+			continue
+		}
+
 		for len(files) > 0 {
 			fi := files[0]
 			files = files[1:]