ci(nix): push Nix builds to a Cachix binary cache

Every Nix build of main and of each release tag now goes to
proton-drive-linux.cachix.org, so installing the flake downloads the
client instead of compiling it. The flake names the cache in nixConfig
for nix run and nix shell, and the install guide shows the nix.settings
a NixOS system build needs, since it does not read an input's
nixConfig. A fork's pull request has no token and only pulls.

docs/DEVELOPMENT.md

@@ -154,7 +154,8 @@ Releases are cut from `main` by pushing a `v*` tag. `.github/workflows/release.y
    `packaging/PKGBUILD` agree;
 2. runs the quality gates;
 3. builds the release binaries and packages a `.tar.gz`, a `.deb` and a GPG-signed `.rpm`, and
-   builds the Nix flake at the tag, which is what a flake input pinned to `vX.Y.Z` gets;
+   builds the Nix flake at the tag, which is what a flake input pinned to `vX.Y.Z` gets, and
+   pushes it to the `proton-drive-linux` Cachix cache (secret `CACHIX_AUTH_TOKEN`);
 4. publishes them as a GitHub release, with the version's section of `docs/CHANGELOG.md` as the
    notes;
 5. calls `.github/workflows/aur.yml`, which updates and pushes the AUR recipes and commits them