flea: run copymanifest's tests on tmpfs

0.3.5's copymanifest::tests::a_garbage_stream_falls_back_... opens its
anonymous (O_TMPFILE) manifest directly in its /tmp test dir, which the
x86_64 builder's /tmp refuses with EOPNOTSUPP. The sibling tests pass
because Writer::create falls back across directories. Same reason the
other filesystem_tests already run on /dev/shm; none of these spawn.

pkgbuilds/flea/PKGBUILD

@@ -116,6 +116,7 @@ check() {
   # identical and undo walks it back. tmpfs allocates inode numbers from a
   # counter and never reuses one, which is what the test assumes.
   local -a filesystem_tests=(
+    backend::copymanifest::tests::
     backend::menu_actions::tests::
     backend::menudelete::tests::
     backend::redo::tests::