fix emacs package-list's "Failed to verify signature..."

Everytime I called M-x package-list-packages, I got

package--check-signature: Failed to verify signature
some_package.el.sig: ("No public key for XXX")

I'm not really sure what effect this had, but it was annoying.


Fix

1. in a random buffer, do

(setq package-check-signature nil) ;; press C-x C-e here

2. do M-x package-list-packages
search for gnu-elpa (complete name: gnu-elpa-keyring-update)
and install that

3. then just restart emacs or do
(setq package-check-signature 'allow-unsigned) ;; press C-x C-e here


The next time you run package-list-packages, the warning should be gone.