release/dist/qnap: preserve .codesigning files as build artifacts

Stop deleting .qpkg.codesigning files in build-qpkg.sh and include
them in the returned artifact list from buildQPKG.

These files contain the last 32 characters of the base64-encoded CMS
signature produced by QDK code signing. They are consumed by pkgserve
to populate <signature> entries in the QNAP repository XML, matching
the format used by myqnap.org and qnapclub.eu.

Updates corp#33203

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
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