Use in-tree builds for PyPy wheels

Doing out-of-tree builds makes a copy that breaks symlinks in the git
tree, causing PyPy wheels to get a version that signifies a dirty tree
when they shouldn't.

This will change in pip soon, but changing this will fix the version for
our release now.

.github/workflows/cibuildwheel.yml

@@ -102,6 +102,7 @@ jobs:
           CIBW_BUILD: "pp37-*"
           CIBW_BEFORE_BUILD: pip install certifi numpy==${{ env.min-numpy-version }}
           CIBW_ARCHS: ${{ matrix.cibw_archs }}
+          PIP_USE_FEATURE: in-tree-build
         if: matrix.cibw_archs != 'aarch64'
 
       - name: Validate that LICENSE files are included in wheels