fix: allow symlink collision error to be downgraded (#369)

Introduces an attr allowing the symlink collision errors to be
downgraded to a warning or ignored.

Closes #358

---

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes
- Breaking change (forces users to change their own code or config): yes
- Suggested release notes appear below:

Allow the "conflicting symlinks" error when creating a venv to be
downgraded to a warning and ignored.

### Test plan

- Manual testing; please provide instructions so we can reproduce:

Run `//py/tests/virtual/django:manage` with the `pyqt6` packages
installed.

---------

Co-authored-by: Alex Eagle <[email protected]>

.github/workflows/ci.yaml

@@ -41,9 +41,10 @@ jobs:
       - uses: actions/checkout@v4
       - run: patch --dry-run -p1 < .bcr/patches/*.patch
 
-  test-release:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - working-directory: e2e/use_release
-        run: ./minimal_download_test.sh
+  # TODO(alex): enable after next release. Matt will yell at you if you forget.
+  # test-release:
+  #   runs-on: ubuntu-latest
+  #   steps:
+  #     - uses: actions/checkout@v4
+  #     - working-directory: e2e/use_release
+  #       run: ./minimal_download_test.sh