ci(iso-build): use new build_iso.sh path after move

The script was relocated to test_tooling/mkarchiso/ in this PR, but
iso-build.yaml still invoked it at the old root path, breaking the
workflow with `./build_iso.sh: No such file or directory` (exit 127).

CWD stays at the repo root since the script does `cp -r .` into
/tmp/archlive/airootfs/... — only the invocation path needs updating.

.github/workflows/iso-build.yaml

@@ -32,7 +32,7 @@ jobs:
       - run: cat /etc/os-release
       - run: pacman-key --init
       - run: pacman --noconfirm -Sy archlinux-keyring
-      - run: ./build_iso.sh
+      - run: ./test_tooling/mkarchiso/build_iso.sh
       - uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: Arch Live ISO