scripts: signall.sh store usign.key as usign.sec

scripts: signall.sh store usign.key as usign.sec

The signall.sh script signs all files inside a tar via GPG and
signify-openbsd (similar to usign)e and attaches the signatures to the
same archive.

Using more recent versions of signify-openbsd requires a specific naming
schema for keys, private ending with .sec and public with .pub. This was
introduced at 763e1148f68f03cb2fa85d022500acf8c66af222[0].

This patch renames the stored key as usign.sec instead of usign.key. As
of the temporary nature of the key storing, this very unlikely breaks
any existing setups.

[0]: https://github.com/openbsd/src/commit/763e1148f68f03cb2fa85d022500acf8c66af222

Signed-off-by: Paul Spooren <mail@aparcar.org>
[adjusted commit subject]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>