ci: attach .deb and .rpm packages to releases and nightlies

Built by nfpm from the static musl binaries, with the man page and the
bash, zsh and fish completions. The names carry no version, so
releases/latest/download/soulseek-rs-amd64.deb stays a working link; the
nightly pair replaces itself on the rolling prerelease.

RELEASE.md

@@ -28,6 +28,11 @@ Releases are automated by [release-plz](https://release-plz.dev). You do not run
      | `soulseek-rs-vX.Y.Z-x86_64-pc-windows-msvc.zip`        |                          |
      | `soulseek-rs-vX.Y.Z-aarch64-pc-windows-msvc.zip`       |                          |
 
+   - packages the two Linux binaries as `soulseek-rs-amd64.deb`, `soulseek-rs-arm64.deb`,
+     `soulseek-rs-x86_64.rpm` and `soulseek-rs-aarch64.rpm`, with the man page and the
+     bash, zsh and fish completions. The names carry no version, so
+     `releases/latest/download/soulseek-rs-amd64.deb` is a link that keeps working,
+
    - rewrites `Formula/soulseek-rs.rb` in the `michel/homebrew-tap` repository so
      `brew install michel/tap/soulseek-rs` picks up the new macOS and Linux archives,
    - merges `master` back into `develop`, so the branch the work happens on carries the
@@ -44,9 +49,15 @@ conflict.
 
 `.github/workflows/nightly.yml` builds the tip of `develop` at 02:17 UTC every
 day. It resolves the commit once, builds the same six targets as a numbered
-release, verifies all twelve archives and checksum files as one set, then
-replaces the mutable `nightly` prerelease and tag. The published release is
-smoke-tested through both the curl and wget installer paths.
+release, packages the two Linux ones as `soulseek-rs-nightly-*.deb` and `.rpm`,
+verifies all sixteen files as one set, then replaces the mutable `nightly`
+prerelease and tag. Every nightly binary reports `X.Y.Z+git<commit time, UTC>.<sha>`
+instead of the crate version. The published release is smoke-tested through the
+curl and wget installer paths, and by installing the `.deb` and the `.rpm`.
+
+Never enable GitHub's Immutable Releases on this repository: nightly replaces
+its assets and moves its tag, and a tag name burnt by that setting cannot be
+reused.
 
 Use **Actions → Nightly → Run workflow** to force a build. The workflow always
 checks out `develop`, regardless of the ref selected in the dispatch form. The