Add --user flag

#232 is failing with a "This
environment is externally managed" error from pip because it's trying to
install to the system Python.

For some reason this doesn't happen on a fork, e.g.,
bpcreech#2. I assume this is due to a
Github probably runner change; I'm not sure whether intentional.

Anyway we can just be explicit; pip should just use a user install during the
test run.

.github/workflows/tests.yaml

@@ -41,7 +41,7 @@ jobs:
         uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.pyver }}
-      - run: pip install 'pip >=23' build
+      - run: pip install --user 'pip >=23' build
       - run: python -m build
       - run: pip install dist/localstripe-*.tar.gz
       - run: python -m localstripe &