setup.sh: use six `X` characters in `mktemp` invocation

Closes #178625

The `busybox` version of `mktemp` requires exactly six `X` characters
in the argument to `mktemp`, unlike the `coreutils` version of `mktemp`.

Let's accomodate packages, like `epson-escpr2`, which fool `setup.sh`
into using the `busybox` version instead of the `stdenv` version.

pkgs/stdenv/generic/setup.sh

@@ -1026,7 +1026,7 @@ configurePhase() {
         # scripts with vendored libtool code.  Preserve mtimes to
         # prevent some packages (e.g. libidn2) from spontaneously
         # autoreconf'ing themselves
-        CONFIGURE_MTIME_REFERENCE=$(mktemp configure.mtime.reference.XXX)
+        CONFIGURE_MTIME_REFERENCE=$(mktemp configure.mtime.reference.XXXXXX)
         find . \
           -executable \
           -type f \