Do load the shared ExtraSettings.php file

When migrating the Jenkins workspace to /srv/ssd , the globally shared
ExtraSettings.php file ended up not being loaded anymore.  This patch
hardcode the path /var/lib/jenkins/jobs/_shared/ExtraSettings.php

Side effect: that will actually enable all of the settings in that file
as soon as this change is deployed.

Settings:
---------

// Debugging: PHP
error_reporting( -1 );
ini_set( 'display_errors', 1 );

// Debugging: MediaWiki
$wgDevelopmentWarnings = true;
$wgShowExceptionDetails = true;

// gallium.wikimedia.org has tmpfs installed, use that instead of
// the default /tmp.
$jenkinsTmpFs = '/var/lib/jenkins/tmpfs';
$jenkinsJobName = getenv( 'JOB_NAME' );
if ( $jenkinsJobName && is_dir( "$jenkinsTmpFs/$jenkinsJobName" ) ) {
	$wgTmpDirectory = "$jenkinsTmpFs/$jenkinsJobName";
}

Change-Id: I2b48959a46667532928129db0aaec98105a682e3