Avoid cp -v in fixtures for portability

This change removes the `-v` option from the one `cp` invocation in
`gix-dir/tests/fixtures/many.sh`, the one fixture script that uses
it.

This is roughly analogous to the preceding removal of `-v` from the
one `mv` invocation that used that, and both fixture scripts are
heavily used due to being used in multiple tests and due to being
intentionally re-run even when `GIX_TEST_IGNORE_ARCHIVES` is not
set.

But while `rm -v` is standardized but only recently, `cp -v` is not
standard. Even POSIX 1003.1-2024 does not require `cp` to recognize
`-v`:

- https://pubs.opengroup.org/onlinepubs/9799919799/utilities/cp.html

See the preceding commit for general considerations, advantages and
disadvantages, and alterntive approaches. As there, this currently
just drops the `-v` without replacing it with anything, based on
the idea that printing the list of files being operated on (here,
copied) may not be important.

- That assumption may be less well-founded here, since the affected
  command, introduced in 482d6f3 (GitoxideLabs#1285), does *not* have other
  nearby `cp` commands that are conceptually related but omit `-v`.

- But, unlike there, it may be reasonable to consider the `-v` here
  to be a (minor) bug, since it is not standardized but it appears
  in test fixture code that seems intended to run all targets.

Taken together, this and the preceding commit allow 106 formerly
failing tests to pass, on the OmniOS system I used for testing.

Author: EliahKagan

gix-dir/tests/fixtures/many.sh

@@ -280,7 +280,7 @@ git init expendable-and-precious-nested-in-ignored-dir
   echo 'ignored/' > .gitignore
   git add .gitignore && git commit -m "init"
   mkdir -p ignored/other
-  cp -Rv ../expendable-and-precious ignored/d
+  cp -R ../expendable-and-precious ignored/d
   rm -Rf ignored/d/*-by-filematch ignored/d/some-*
   mkdir -p other/ignored && >other/ignored/a
 )