test setup: do not set the sticky bit on a regular file

sticky bit only has a function on directories.
openbsd does not let one set sticky on files.
other systems seem to just ignore it.

borg/testsuite/archiver.py

@@ -162,7 +162,7 @@ class ArchiverTestCase(ArchiverTestCaseBase):
         # Directory
         self.create_regular_file('dir2/file2', size=1024 * 80)
         # File mode
-        os.chmod('input/file1', 0o7755)
+        os.chmod('input/file1', 0o6755)
         # Hard link
         os.link(os.path.join(self.input_path, 'file1'),
                 os.path.join(self.input_path, 'hardlink'))