feat: the ISO's boot menu and /etc/steammachine-iso-build name its release tag too (were the build date): the tag decides everything

CHANGELOG.md

@@ -34,7 +34,7 @@ Releases: GitHub creates the tag and a release with the notes; the ISO itself (o
 **Features:**
 
 * **ISO:** named after its release tag (`steamify-cachyos-2.9.3-2026.09.29-2230-x86_64.iso`; built by hand, the date), labelled after the Steamify version on it and the build time (`STEAMIFY_2_9_1_20260929_1432`, was CachyOS's `COS_<year><month>`); CachyOS Hello installs it like CachyOS's own
-  release (the build date is kept in `/etc/steammachine-iso-build`)
+  release (the ISO's own release, its tag without the v, is in `/etc/steammachine-iso-build` and the boot menu)
 * **Installer (CachyOS - Steamify Installer):**
   * A Steamify page after the packages page: which Steam Machine parts to set up (rows from
     `steamify.sh --defaults --list`, the newest Steamify fetched before Calamares starts) and where

util-iso.sh

@@ -140,7 +140,9 @@ prepare_profile(){
 
     info "Profile: [%s]" "${profile}"
 
-    local _iso_version="$(date +%y%m%d)"
+    # The release tag without its v when the release workflow builds (STEAMIFY_ISO_VERSION): the boot menu
+    # and /etc/steammachine-iso-build name the same release as the file, label and tag. By hand: the date.
+    local _iso_version="${STEAMIFY_ISO_VERSION:-$(date +%y%m%d)}"
     change_grub_version "${_iso_version}"
 
     # Fetch up-to-date version of CachyOS repo mirrorlist