cache: ignore ErrNotExist during cleanup of old files

Two restic processes running concurrently can try to remove the same
files from the cache. This could cause one process to fail with an error
if the other one has already remove a file that the current process also
tries to delete.

internal/cache/file.go

@@ -165,7 +165,8 @@ func (c *Cache) Clear(t restic.FileType, valid restic.IDSet) error {
 			continue
 		}
 
-		if err = fs.Remove(c.filename(backend.Handle{Type: t, Name: id.String()})); err != nil {
+		// ignore ErrNotExist to gracefully handle multiple processes running Clear() concurrently
+		if err = fs.Remove(c.filename(backend.Handle{Type: t, Name: id.String()})); err != nil && !errors.Is(err, os.ErrNotExist) {
 			return err
 		}
 	}