fixup! test workflow: use an explicit matrix variable instead of an inline value

Author: nicoddemus

.github/workflows/test.yml

@@ -247,7 +247,7 @@ jobs:
             tox_env: "doctesting"
             use_coverage: true
 
-    continue-on-error: ${{ fromJSON(matrix.xfail) && true || false }}
+    continue-on-error: ${{ matrix.xfail && true || false }}
 
     steps:
     - uses: actions/checkout@v5