build: Use AlmaLinux for the RHEL packaging image (#8045)

.github/scripts/strategy-matrix/generate.py

@@ -219,7 +219,7 @@ def expand_linux_matrix(linux: LinuxFile, minimal: bool) -> list[MatrixEntry]:
 def expand_linux_packaging(linux: LinuxFile) -> list[PackagingEntry]:
     """Generate the packaging matrix from a LinuxFile's package_configs section.
 
-    Packaging uses vanilla distro images (debian:bookworm, ubi9, …) instead of
+    Packaging uses vanilla distro images (debian:bookworm, almalinux:9) instead of
     the nix-based build images, because deb/rpm tooling (debhelper, rpm-build)
     is taken from the distro's archive rather than from nixpkgs. Each config
     entry carries its own 'image'.

.github/workflows/build-packaging-images.yml

@@ -36,8 +36,9 @@ jobs:
         distro:
           - name: debian
             base_image: debian:bookworm
+          # AlmaLinux rather than UBI9, which does not ship rpm-sign.
           - name: rhel
-            base_image: registry.access.redhat.com/ubi9/ubi:latest
+            base_image: almalinux:9
     uses: XRPLF/actions/.github/workflows/build-multiarch-image.yml@9e7e4e80af9e684c116b38369add8eea64451f32
     with:
       image_name: xrpld/packaging-${{ matrix.distro.name }}