test613: make it pass on Windows, fix postprocess, unignore in CI

test613: make it pass on Windows, fix postprocess, unignore in CI

- on native Windows (also when using MSYS2 openssh), the group and other
  permissions do not end up as requested by Perl's chmod:
  ```diff
  --- log/8/check-expected
  +++ log/8/check-generated
  @@ -1,3 +1,3 @@
   d?????????    N U         U               N ???  N NN:NN asubdir[LF]
  --rw?rw?rw?    1 U         U              37 Jan  1  2000 plainfile.txt[LF]
  +-rw?r-?r-?    1 U         U              37 Jan  1  2000 plainfile.txt[LF]
   -r-?r-?r-?    1 U         U              47 Dec 31  2000 rofile.txt[LF]
  ```
  Ref: https://github.com/curl/curl/actions/runs/14004029192/job/39215359241?pr=16781#step:15:1596
  Fix it by ignoring group and other attributes.

- fix failing postprocess cleanup by making the read-only test file
  writeable again before deleting it. Fixing:
  ```
  Directory not empty at ../../tests/libtest/test613.pl line 83.
  ```
  (seen on Windows with Git for Windows `perl.exe`)

- unignore in GHA/windows.

Closes #16791