ci: fix --precision flag and remove Python 3.14 step in github job

--${{ matrix.precision }} expands to bare '--' when precision is empty,
which is parsed as an argument separator. Restore the PRECISION env var
approach so empty precision is correctly omitted.

Remove the manual 'Set up Python 3.14' step; the lint-gate job already
pins Python 3.12 via setup-python and the github runners use the
system default for the build/test steps.

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

Author: sbryngelson

.github/workflows/test.yml

@@ -137,16 +137,12 @@ jobs:
           printenv | sort > /tmp/env_after
           diff /tmp/env_before /tmp/env_after | grep '^>' | sed 's/^> //' >> $GITHUB_ENV
 
-      - name: Set up Python 3.14
-        uses: actions/setup-python@v5
-        with:
-          python-version: '3.14'
-
       - name: Build
         run:  |
-          /bin/bash mfc.sh test -v --dry-run -j $(nproc) --${{ matrix.debug }} --${{ matrix.mpi }} --${{ matrix.precision }} $TEST_ALL
+          /bin/bash mfc.sh test -v --dry-run -j $(nproc) --${{ matrix.debug }} --${{ matrix.mpi }} $PRECISION $TEST_ALL
         env:
           TEST_ALL: ${{ matrix.mpi == 'mpi' && '--test-all' || '' }}
+          PRECISION: ${{ matrix.precision != '' && format('--{0}', matrix.precision) || '' }}
 
       - name: Test
         run:  |