diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 4f61ab2d9b..2e4a4bc841 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -56,12 +56,12 @@ jobs:
           rm -rf firedrake_venv
       - name: Build Firedrake
         run: |
+          unset PETSC_DIR SLEPC_DIR
           cd ..
           # Linting should ignore unquoted shell variable $COMPLEX
           # shellcheck disable=SC2086
           ./firedrake/scripts/firedrake-install \
             $COMPLEX \
-            --honour-petsc-dir \
             --mpicc="$MPICH_DIR"/mpicc \
             --mpicxx="$MPICH_DIR"/mpicxx \
             --mpif90="$MPICH_DIR"/mpif90 \
@@ -82,6 +82,7 @@ jobs:
             --install defcon \
             --install gadopt \
             --install asQ \
+            --package-branch petsc ksagiyam/merge_upstream \
             || (cat firedrake-install.log && /bin/false)
       - name: Install test dependencies
         run: |
@@ -92,6 +93,7 @@ jobs:
           python -m pip list
       - name: Test Firedrake
         run: |
+          unset PETSC_DIR SLEPC_DIR
           . ../firedrake_venv/bin/activate
           echo OMP_NUM_THREADS is "$OMP_NUM_THREADS"
           echo OPENBLAS_NUM_THREADS is "$OPENBLAS_NUM_THREADS"
@@ -107,6 +109,7 @@ jobs:
       - name: Test pyadjoint
         if: ${{ matrix.scalar-type == 'real' }}
         run: |
+          unset PETSC_DIR SLEPC_DIR
           . ../firedrake_venv/bin/activate
           cd ../firedrake_venv/src/pyadjoint
           python -m pytest \