ci: always rebuild on Phoenix even if rm -rf build leaves partial directory

If stale NFS handles prevent full deletion of build/, the old
'if [ ! -d build ]' guard would skip the rebuild entirely, leaving
stale binaries from a different compute node that could cause SIGILL.
Force a rebuild on Phoenix unconditionally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/common/bench.sh

@@ -29,7 +29,7 @@ if [ "$job_cluster" = "phoenix" ]; then
     rm -rf build 2>/dev/null || true
 fi
 
-if [ ! -d "build" ]; then
+if [ "$job_cluster" = "phoenix" ] || [ ! -d "build" ]; then
     source .github/scripts/retry-build.sh
     retry_build ./mfc.sh build -j $n_jobs $build_opts || exit 1
 fi

.github/workflows/common/test.sh

@@ -17,7 +17,9 @@ if [ "$job_cluster" = "phoenix" ]; then
     rm -rf build 2>/dev/null || true
 fi
 
-if [ ! -d "build" ]; then
+# Phoenix must always rebuild (heterogeneous compute nodes → ISA mismatch risk),
+# even if rm above left a partial build/ directory behind.
+if [ "$job_cluster" = "phoenix" ] || [ ! -d "build" ]; then
     source .github/scripts/retry-build.sh
 
     # Phoenix: smoke-test the syscheck binary to catch architecture mismatches