installer: Don't shell out if it's disabled

Bug: T191947
Change-Id: I16a82d271157cd0024aa14d7eaec80b4870947b5

includes/installer/Installer.php

@@ -990,6 +990,10 @@ abstract class Installer {
 			return true;
 		}
 
+		if ( Shell::isDisabled() ) {
+			return true;
+		}
+
 		# Get a list of available locales.
 		$result = Shell::command( '/usr/bin/locale', '-a' )
 			->execute();

includes/utils/ExecutableFinder.php

@@ -94,6 +94,11 @@ class ExecutableFinder {
 	 * @return bool|string
 	 */
 	public static function findInDefaultPaths( $names, $versionInfo = false ) {
+		if ( Shell::isDisabled() ) {
+			// If we can't shell out, there's no point looking for executables
+			return false;
+		}
+
 		$paths = self::getPossibleBinPaths();
 		foreach ( (array)$names as $name ) {
 			foreach ( $paths as $path ) {