--with-prrte=internal is ignored if a valid external prrte is available

configure option `--with-prrte=internal` is ignored when an external, valid PRRTE version is found in PATH etc. 

## Background information

### What version of Open MPI are you using? (e.g., v4.1.6, v5.0.1, git branch name and hash, etc.)

Visible in `main` and `v5.x`

### Fix

This is a simple error in `config/ompi_setup_prrte.m4`, fix incoming

