use MBL tag instead of branch, and clean up call to install it

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         os: [ubuntu-latest, windows-latest]
         python-version: ["3.10", "3.11"]
         test_env: [python, docs, mypy]
-        mbl_branch: [maint_9.1.x]
+        mbl_tag: [v9.1.1]
         exclude:
           # only test mypy on linux for all versions of python
           - os: windows-latest
@@ -59,17 +59,14 @@ jobs:
             mv modelicafmt '/c/Program Files/'
           fi
       - name: Install MBL
-        env:
-          MATRIX_OS: ${{ matrix.os }}
-          MBL_BRANCH: ${{ matrix.mbl_branch }}
         run: |
-          if [[ "${MATRIX_OS}" == 'ubuntu-latest' ]]; then
+          if [[ '${{ matrix.os }}' == 'ubuntu-latest' ]]; then
             MODELICAPATH='/home/runner/work/modelica-buildings'
           else
             echo $GITHUB_WORKSPACE
             MODELICAPATH='/c/Program Files/modelica-buildings'
           fi
-          git clone --single-branch --branch ${MBL_BRANCH} https://github.com/lbl-srg/modelica-buildings.git "${MODELICAPATH}"
+          git clone --single-branch --branch ${{ matrix.mbl_tag }} https://github.com/lbl-srg/modelica-buildings.git "${MODELICAPATH}"
           cd "${MODELICAPATH}"
           echo "Git branch is $(git branch)"
           # export MODELICAPATH for subsequent steps