TEST BRANCH ONLY -- do not merge: point CI at the KEnRef DD-guard branch

Temporarily repoints KENREF_VERSION from the v2.0.0 tag to the
worktree-kenref-dd-guard branch, so the fork CI builds the KEnRef commit that
adds the domain-decomposition guard to KEnRefBias.

Purpose: the local verification build came up with MPI off, so it could not
exercise comm.Get_size() > 1. The fork CI has real -mpi- and -debug-mpi-
variants, which is what this branch is for. Revert before any merge.

Author: aalhossary

.ci/install.kenref

@@ -9,7 +9,7 @@ set -x
 # to be on a branch today, and a pinned tag also lets the install directory be cached.
 # KENREF_VERSION and KENREF_URL are parsed out of this file by the workflow (to resolve the ref to a
 # commit for the cache key), so keep them as plain, unquoted assignments at the start of a line.
-KENREF_VERSION=v2.0.0
+KENREF_VERSION=worktree-kenref-dd-guard
 KENREF_URL=https://github.com/Smith-Group/KEnRef.git
 KENREF_INSTALL_DIR="$HOME/opt/kenref"