jquery: fix reproducible build issue for PREMIRRORS downloads

jquery: fix reproducible build issue for PREMIRRORS downloads

The sources of jquery are pure text files:
...
SRC_URI = "\
    https://code.jquery.com/${BP}.js;name=js;subdir=${BP} \
    https://code.jquery.com/${BP}.min.js;name=min;subdir=${BP} \
    https://code.jquery.com/${BP}.min.map;name=map;subdir=${BP} \
    "
...

When use git repo to save them and clone locally as PREMIRRORS for
bitbake. In this case, mtime of these files will be current time
when checking out. For this recipe, source_date_epoch will get by
function get_source_date_epoch_from_youngest_file, mtime of the
source files are not stable, so cause the generated package not
reproducible.

Due to the mtime of source files in upstream is always 1991-10-18,
even for the new version
$ wget https://code.jquery.com/jquery-3.0.0.js
$ ls --full-time jquery-3.0.0.js
-rw-r--r-- 1 hjia users 263268 1991-10-18 20:00:00.000000000 +0800 jquery-3.0.0.js

$ wget https://code.jquery.com/jquery-3.7.1.js
$ ls --full-time jquery-3.7.1.js
-rw-r--r-- 1 hjia users 285314 1991-10-18 20:00:00.000000000 +0800 jquery-3.7.1.js

Then explicitly set above timestamp to SOURCE_DATE_EPOCH

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>