ci(release): build the Nix flake at every release tag

A flake input pinned to vX.Y.Z is the Nix release, so the release
workflow now builds the flake at the tag and checks its version before
the GitHub release is published. The install guide shows how to pin the
input to a release tag, and the release checklist names the files that
carry the version.

docs/DEVELOPMENT.md

@@ -153,15 +153,18 @@ Releases are cut from `main` by pushing a `v*` tag. `.github/workflows/release.y
 1. checks that the tag, the workspace version in `Cargo.toml` and `pkgver` in
    `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`;
-4. publishes them as a GitHub release;
+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;
+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
    back to `main`.
 
 To prepare a release:
 
 ```bash
-# 1. Bump the version in Cargo.toml and packaging/PKGBUILD, then refresh the lock file.
+# 1. Bump the version in Cargo.toml, packaging/PKGBUILD, po/pdfs.pot and the pinned flake
+#    URL in docs/INSTALL.md, then refresh the lock file.
 cargo update --workspace
 # 2. Move the [Unreleased] notes in docs/CHANGELOG.md under the new version.
 # 3. Commit, tag and push.