Fix sdist usage for master

Author: connorjward

.github/workflows/core.yml

@@ -179,6 +179,7 @@ jobs:
           pip cache remove slepc4py
 
           if [ ${{ inputs.target_branch }} = 'release' ]; then
+            EXTRA_BUILD_ARGS=''
             EXTRA_PIP_FLAGS=''
           else
             : # Install build dependencies
@@ -192,12 +193,13 @@ jobs:
             pip install --no-deps ngsPETSc netgen-mesher netgen-occt
 
             : # We have to pass '--no-build-isolation' to use a custom petsc4py
+            EXTRA_BUILD_ARGS='--no-isolation'
             EXTRA_PIP_FLAGS='--no-build-isolation'
           fi
 
           : # Install from an sdist so we can make sure that it is not ill-formed
           pip install build
-          python -m build ./firedrake-repo --sdist
+          python -m build ./firedrake-repo --sdist "$EXTRA_BUILD_ARGS"
 
           pip install --verbose $EXTRA_PIP_FLAGS \
             --no-binary h5py \