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

package/install-packaging-tools.sh

@@ -28,6 +28,7 @@ esac
 #   - debhelper and dpkg-dev build the DEB
 #   - rpm-build builds the RPM, with systemd-rpm-macros and redhat-rpm-config
 #     supplying the systemd and find-debuginfo macros the spec uses
+#   - rpm-sign signs the built RPM
 #   - git gives build_pkg.sh a real history to read SOURCE_DATE_EPOCH from;
 #     without one the timestamp falls back to the wall clock
 #   - curl uploads the finished packages in publish_pkg.sh
@@ -50,6 +51,7 @@ function install() {
                 curl-minimal \
                 git \
                 rpm-build \
+                rpm-sign \
                 redhat-rpm-config \
                 systemd-rpm-macros
             ;;