matrix: fix format string to build arm images

problem: {d} was not getting expanded, breaking arm64 builds and
manifest generation

Author: trws

src/test/generate-matrix.py

@@ -159,7 +159,7 @@ def __str__(self):
     for d in ("bookworm", "noble", "fedora40", "el9"):
         matrix.add_build(
             name=f"{d} - arm64",
-            image="{d}",
+            image=f"{d}",
             platform="linux/arm64",
             docker_tag=True,
             command_args="--install-only ",