CI: Fix usage of soon-to-be obsolete items

[bcc32]

if-let, when-let, and gensym are obsolete starting in Emacs 31.1. This commit fixes the Emacs snapshot CI build, but unfortunately needs to drop support for Emacs 25.

[bcc32]

@purcell, do you think it is reasonable to drop support for Emacs 25? Emacs 26 was released in 2018. I suppose we could also wait until Emacs 31 is actually released before we need to do this, but it would be a little unfortunate to have the snapshot CI continually failing until then. (Is it possible to suppress the snapshot CI failure emails?)

[purcell]

Sorry, I hadn't seen this, and fixed it myself recently. 🤦‍♂️