shred: fix permissions_set_readonly_false clippy error

[miles170]

Closes #4487

[tertsdiepraam]

I have only sets the write access for the owner on UNIX platform, not sure should it sets to group or others?

You can test it:

❯ ls -l test
---------- 1 terts terts 77824 10 mrt 11:29 test
❯ shred test
shred: test: failed to open for writing: Permission denied
❯ shred test -f
❯ ls -l test
--w------- 1 terts terts 77824 10 mrt 11:31 test

For some more evidence, here's the strace output that I think is responsible:

openat(AT_FDCWD, "test", O_WRONLY|O_NOCTTY) = -1 EACCES (Permission denied)
chmod("test", 0200)                     = 0
openat(AT_FDCWD, "test", O_WRONLY|O_NOCTTY) = 3

In contrast, here's strace for the current uutils main branch:

chmod("test", 0100666)                  = 0

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/timeout. tests/misc/timeout is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?