This is used to implement the --installroot option for installation. In earlier
PEAR versions, this was implemented in PEAR_Installer::install(),
but this makes it more difficult to track with the introduction of channels, and to
satisfy better encapsulation, it has been moved to PEAR_Config.
Pass in a full path. On retrieving any directory configuration variable, the
value will be prepended with the installroot specified in this method. For example,
if php_dir is /usr/lib/php, and
setInstallRoot() is used with /hoopla/boo,
the value returned from get() would be
/hoopla/boo/usr/lib/php. Use an empty string ''
to reset an installroot.