ci: Build cfsctl with rhel9 feature for CentOS Stream 10 compatibility

CentOS Stream 10's kernel 6.12 doesn't support direct file-backed
EROFS mounts (added post-6.12 in mainline). The rhel9 feature enables
loopback device creation for EROFS images, which is needed for the
composefs overlay mount to work.

Assisted-by: OpenCode (Claude Opus 4)

Author: cgwalters

.github/workflows/build-cfsctl-artifact.yml

@@ -27,7 +27,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
 
       - name: Build cfsctl (release)
-        run: cargo build --release -p cfsctl --features composefs/pre-6.15,composefs-oci/oci-client
+        run: cargo build --release -p cfsctl --features composefs/rhel9,composefs-oci/oci-client
 
       - name: Install oras CLI
         run: |