Move snapshotPathPrefix into a method

This allows the value to be determined on the fly, which supports consumers that use Kaniko snaphot as a library and may need to change the value of config.KanikoDir

pkg/snapshot/snapshot.go

@@ -33,7 +33,7 @@ import (
 )
 
 // For testing
-var snapshotPathPrefix = config.KanikoDir
+var snapshotPathPrefix = ""
 
 // Snapshotter holds the root directory from which to take snapshots, and a list of snapshots taken
 type Snapshotter struct {
@@ -119,7 +119,7 @@ func (s *Snapshotter) TakeSnapshot(files []string, shdCheckDelete bool) (string,
 // TakeSnapshotFS takes a snapshot of the filesystem, avoiding directories in the ignorelist, and creates
 // a tarball of the changed files.
 func (s *Snapshotter) TakeSnapshotFS() (string, error) {
-	f, err := ioutil.TempFile(snapshotPathPrefix, "")
+	f, err := ioutil.TempFile(s.getSnashotPathPrefix(), "")
 	if err != nil {
 		return "", err
 	}
@@ -138,6 +138,13 @@ func (s *Snapshotter) TakeSnapshotFS() (string, error) {
 	return f.Name(), nil
 }
 
+func (s *Snapshotter) getSnashotPathPrefix() string {
+	if snapshotPathPrefix == "" {
+		return config.KanikoDir
+	}
+	return snapshotPathPrefix
+}
+
 func (s *Snapshotter) scanFullFilesystem() ([]string, []string, error) {
 	logrus.Info("Taking snapshot of full filesystem...")