mkosi: update arch commit reference

mkosi: update arch commit reference

* d74b24c7c6 meson options: vcs-tag=false
* 7424fedef0 upgpkg: 256.1-1: new upstream release
* 6016864c99 upgpkg: 256-3: rebuild to fix man pages...
* 6f6d48b221 upgpkg: 256-2: update shells
* b9ce44e766 drop systemd-stable repository, build from main one
* baf4803252 upgpkg: 256-1: new upstream release
* 1d75e7ddaa add an install hint on NEWS
* 6f17a23127 Ups, inverted...
* d76029f7cc update meson options
* 6e7929dd98 upgpkg: 256rc4-1: new upstream pre-release - testing only!
* 7b70e7b0e7 upgpkg: 256rc3-2: fast-forward to current git main, non-official testing
* a438b879e2 upgpkg: 256rc3-1: new upstream pre-release, non-official testing
* ff30a600c9 upgpkg: 256rc2-1: new upstream pre-release, non-official testing