Drop zopfli from Rocky 9 test case.

It’s technically available via EPEL, but it’s also likely fine to just
skip that case instead of dealing with pulling in EPEL given that the
packaged version is functionally identical across the tested distros.

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 738ae87..64de82c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -38,7 +38,7 @@
       if: matrix.container == 'rockylinux:9'
       run: |
         dnf groupinstall -y "Development Tools" "RPM Development Tools"
-        dnf install -y zstd binutils binutils-devel gnupg2 pbzip2 openssl zopfli
+        dnf install -y zstd binutils binutils-devel gnupg2 pbzip2 openssl
         
     - name: Install Alpine packages
       if: matrix.container == 'alpine:latest'