tests: Update generated PKG-INFO fixture

Since commit f5cbba2b (#4766),
test_executor_should_write_pep610_url_references_for_git has been
writing a different version of this file than the one checked into the
repository, leaving a dirty working tree.  Fix it and add a CI test.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

.github/workflows/main.yml

@@ -111,3 +111,8 @@ jobs:
 
       - name: Run pytest (poetry-plugin-export)
         run: poetry run python -m pytest -p no:sugar -q poetry-plugin-export/tests/
+
+      - name: Check for clean working tree
+        run: |
+          git diff --exit-code --stat HEAD
+          git -C poetry-plugin-export diff --exit-code --stat HEAD

tests/fixtures/git/github.com/demo/demo/demo.egg-info/PKG-INFO

@@ -6,9 +6,5 @@ Home-page: https://github.com/demo/demo
 Author: Sébastien Eustace
 Author-email: sebastien@eustace.io
 License: MIT
-Platform: UNKNOWN
 Provides-Extra: foo
 Provides-Extra: bar
-
-UNKNOWN
-