gpg 2 --list-keys output format incompatible with parsing

Versions:
- keychain: 2.8.2-0.1
- gnupg 2.2.4-1ubuntu1

$ keychain --agents gpg B269215B

 * keychain 2.8.2 ~ http://www.funtoo.org
 * Starting gpg-agent...
 * Warning: can't find B269215B; skipping

If you run `gpg --list-secret-keys` on the version of gnupg in bionic you can see the problem. The output now prefers a multi-line listing for the key ids, before it was 2048R/B269215B (for example).

Inspecting the source, it's coming down to line 1088 (in ubuntu/debian version 2.8.2-0.1).

    pm_gpgsecrets="`gpg --list-secret-keys 2>/dev/null | cut -d/ -f2 | cut -d' ' -f1 | xargs`"

Which needs a:

    --keyid-format short

in the command to be compatible with the simple cut parsing of '-d/'

After this addition, the keychain program behaves as normal.