handle _man_dir writabililty more robustly

This ensures that mismatched permissions/ownership on `${_man_dir}` and `${_man_dir}/man1` don't cause the script to fail.

*If* `${man_dir}/man1` already exists and is writable, OR `${_man_dir}/man1` *doesn't* yet exist, but `${_man_dir}` is
writable, then the script continues without sudo.

If `/man1` doesn't exist, and it's parent isn't writable, we need sudo.
If `/man1` *does* exist, and it's not writable, we need sudo.

install.sh

@@ -150,7 +150,8 @@ main() {
 
     # After this point, $sudo is used for _bin_dir instead of _bin_dir
 
-    if test_writeable "${_man_dir}"; then
+    if { [ -d "${_man_dir}/man1" ] && test_writeable "${_man_dir}/man1}"; } ||
+        { ! [ -d "${_man_dir}/man1" ] && test_writeable "${_man_dir}"; }; then
         log "Installing zoxide man pages, please wait…"
         _sudo=""
     else