Add lsst-ts-xml to build.yaml for test coverage.

This way we can track that everything works when there is a conda package using
the lsst namespace.

.github/workflows/build.yaml

@@ -45,6 +45,11 @@ jobs:
         run: |
           mamba install numexpr
 
+      - name: Install lsst-ts-xml for test coverage of installing into lsst namespace
+        shell: bash -l {0}
+        run: |
+          mamba install lsst-ts-xml
+
       - name: Install dependencies
         shell: bash -l {0}
         run: |