Publish builds to a branch instead of to releases.

Releases and version numbers are gone. CI now rebuilds all three platforms
on every change to main and commits the packages to the `builds` branch,
so what anyone downloads is the current state of the repository. The
branch is rewritten from scratch each time rather than added to, so the
repository keeps one copy of the packages instead of a ~70 MB set per
commit.

Without tags there is no version to compare, so a build carries the
commit it was made from and the branch carries a VERSION file naming the
same commit. The updater fetches those few bytes and treats any
difference as being behind; commits are not ordered, and the branch only
ever holds the current build.

The installers fetch from the branch over raw.githubusercontent.com for
the same reason.

Co-authored-by: Cursor <cursoragent@cursor.com>

installers/maturita-installer-linux.sh

@@ -1,9 +1,9 @@
 #!/bin/sh
 # maturita.c installer for Linux.
 #
-# Carries no application of its own: it downloads whatever the newest release
-# is at the moment you run it, so this file never goes stale. GitHub resolves
-# the /releases/latest/download/ path to the current tag on every request.
+# Carries no application of its own: it downloads the current build straight
+# from the repository's `builds` branch, which CI rewrites whenever main
+# changes. This file therefore never goes stale.
 #
 # Run it with:  sh maturita-installer-linux.sh
 # The app updates itself from then on.
@@ -11,8 +11,9 @@
 set -eu
 
 REPO="pepaskopec-blip/maturita.c"
+BRANCH="builds"
 ASSET="maturita-linux-x86_64.AppImage"
-URL="https://github.com/$REPO/releases/latest/download/$ASSET"
+URL="https://raw.githubusercontent.com/$REPO/$BRANCH/$ASSET"
 
 # ~/Applications is where desktop environments look for AppImages, and the app
 # keeps its progress/ directory next to its own file.