new-installer: Use `` instead of wfShellExec() like the old installer

new-installer: Use `` instead of wfShellExec() like the old installer

This avoids runnin into the memory limitation of ulimit.sh when
auto-detecting if programs exist during installation. On my 64 bit
Debian system calling e.g. diff3 --version takes more than the default
10MB memory limit.

Can be reproduced as:

    <?php
    require 'maintenance/commandLine.inc';
    $var = wfShellExec("/usr/bin/diff3 --version 2>&1");
    echo "<$var>\n";
    ?>

Which on my system gives you:

    php test.php
    /home/avar/g/phase3/bin/ulimit4.sh: xmalloc: ../bash/variables.c:3446: cannot allocate 1270 bytes (53248 bytes allocated)
    <>

The old installer just did ``, revert to this behavior until we raise
the default memory limit, or find some workaround for this issue.