fix: never reuse a backup folder of another run

When every dated name in retrobios-backup already existed, the loop ended on the last one and wrote into it. It now raises, so the next location is tried and the file stays in place if none works. A holder another user owns is refused on systems with user ids.

tests/test_install.py

@@ -5218,6 +5218,29 @@ class AFileYouHadIsKept(unittest.TestCase):
         self.assertEqual(len(inside), 1)
         self.assertEqual((inside[0] / "x.bin").read_bytes(), self.OLD)
 
+    def test_a_holder_with_every_name_taken_is_not_reused(self):
+        # Folders planted under every dated name of this run: none of them
+        # may receive the user's file.
+        holder = self.home / install.BACKUP_DIR
+        stamp = "20260101-000000"
+        for n in range(1, 1000):
+            (holder / (stamp if n == 1 else f"{stamp}-{n}")).mkdir(parents=True)
+        (self.bios / "x.bin").write_bytes(self.OLD)
+        with unittest.mock.patch.object(install.time, "strftime", return_value=stamp):
+            self.assertTrue(self._download("x.bin")[1])
+        self.assertEqual(sum(1 for p in holder.rglob("x.bin")), 0)
+        inside = list((self.bios / install.BACKUP_DIR).iterdir())
+        self.assertEqual(len(inside), 1)
+        self.assertEqual((inside[0] / "x.bin").read_bytes(), self.OLD)
+
+    @unittest.skipUnless(hasattr(os, "getuid"), "user ids are POSIX")
+    def test_a_holder_another_user_owns_is_refused(self):
+        (self.home / install.BACKUP_DIR).mkdir()
+        with self.assertRaises(OSError):
+            with unittest.mock.patch.object(install.os, "getuid", return_value=os.getuid() + 1):
+                install._check_holder(self.home / install.BACKUP_DIR)
+        install._check_holder(self.home / install.BACKUP_DIR)
+
     def test_restore_refuses_a_folder_the_installer_did_not_write(self):
         planted = self.home / install.BACKUP_DIR / "20260101-000000"
         (planted / "system").mkdir(parents=True)