proper home directory for ssh configuration (e.g. ~/.ssh/config);
avoid redundant mkpasswd + mkgroup information, as recommended by Cygwin documentation;

--- a/Admin/Windows/Cygwin/isabelle/postinstall	Thu Oct 06 14:40:45 2022 +0200
+++ b/Admin/Windows/Cygwin/isabelle/postinstall	Thu Oct 06 17:24:55 2022 +0200
@@ -4,8 +4,7 @@
 
 bash /etc/postinstall/base-files-mketc.sh.done
 
-mkpasswd -l >/etc/passwd
-mkgroup -l >/etc/group
+mkpasswd -l -u "$USER" -p "$(cygpath -u "$HOMEDRIVE$HOMEPATH\\..")" >/etc/passwd
 
 find -type d -exec setfacl -m default:user::rwx,default:group::r-x,default:other:r-x '{}' +
 find -type d -exec chmod 755 '{}' +