Honor RESTIC_CACHE_DIR environment variable

Fix #3382: restic check doesn't obey the RESTIC_CACHE_DIR environment variable

internal/cache/dir.go

@@ -6,10 +6,15 @@ import (
 	"path/filepath"
 )
 
+// EnvDir return $RESTIC_CACHE_DIR env
+func EnvDir() string {
+	return os.Getenv("RESTIC_CACHE_DIR")
+}
+
 // DefaultDir returns $RESTIC_CACHE_DIR, or the default cache directory
 // for the current OS if that variable is not set.
 func DefaultDir() (cachedir string, err error) {
-	cachedir = os.Getenv("RESTIC_CACHE_DIR")
+	cachedir = EnvDir()
 	if cachedir != "" {
 		return cachedir, nil
 	}