firefox-overlay: expose more for flakes overlay/users

[colemickens]

This is sort of an experiment for now until I get more feedback.

This is a small tweak that allows me to consume this overlay in flake-firefox-nightly and expose a --pure-eval-able firefox-nightly attribute for flakes users that want to pin everything and have a purely eval-able system.

Fundamentally this exposes functions so that an outside caller can determine latest information and checksums and then re-instantiate that exact version purely later. (See update.sh in flake-firefox-nightly for more of an idea of how it works.)

[colemickens]

Note to self: this doesn't quite look backward compatible and/or the other invocations of firefoxVersion need updating.

[nbp]

May I suggest adding an optional info attribute as an optional attribute expected by versionInfo, which if set would be returned without checking, and is expected to be the content produced by versionInfo if not set.

This way this would be backward compatible.

Also I am not sure about the added value of having Firefox nightly as a flake knowing that the update script would have to be executed twice per day in order to follow the latest version. Which defeat the point of using Nightly at all.

[nbp]

@colemickens This change sounds good and backward compatible as well.
Do you still have modification to do, or should I proceed and merge these changes?

[colemickens]

I have no more changes to make, it would be great to have this merged. Thanks for the suggestion!

[colemickens]

I'm trying to extend my flake to expose the other firefox versions, but they work slightly different. Due to how the verification works for actual releases, the signature check happens "later" after my impure eval has run. (versus the nightly build, where all needed checksums are determined during the impure eval and stashed).

basically these two fetchurl calls:

nixpkgs-mozilla/firefox-overlay.nix

Lines 116 to 117 in efda5b3

	file = builtins.fetchurl info.chksum;
	asc = builtins.fetchurl info.chksumSig;

cc: @cole-h

[nbp]

Thanks!