We occasionally see races over the lockfile used by externalsrc/devtool
when walking files for the source_date_epock calculation. Skip this file
if present to avoid the issues and fix a real issue where SDE could be
contaminated too.
[YOCTO #14921]
(From OE-Core rev: 4bc0eb4bd90e6e6e46581a8ed367212bdd910a26)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>