MDL-68558 admin: Allow to go back to plugins check page on failed deps

Once the admin reaches the Plugin dependencies check with a failed
plugin dependency, the "Continue" button should take him/her to the
previous page where the dependency can be sorted out, or the plugin
installation cancelled. Also the "Cancel this installation" should work
on this page, too.

None of this was happening correctly because the page with failed
dependencies is loaded with "confirmplugincheck" set to 1 as a result
of confirming the previous step. We must explicitly override it back to
0 to navigate the user back to the previous step.

admin/renderer.php

@@ -1110,7 +1110,7 @@ class core_admin_renderer extends plugin_renderer_base {
 
                 if (!empty($installabortable[$plugin->component])) {
                     $status .= $this->output->single_button(
-                        new moodle_url($this->page->url, array('abortinstall' => $plugin->component)),
+                        new moodle_url($this->page->url, array('abortinstall' => $plugin->component, 'confirmplugincheck' => 0)),
                         get_string('cancelinstallone', 'core_plugin'),
                         'post',
                         array('class' => 'actionbutton cancelinstallone d-block mt-1')
@@ -1209,7 +1209,7 @@ class core_admin_renderer extends plugin_renderer_base {
 
         if ($installabortable) {
             $out .= $this->output->single_button(
-                new moodle_url($this->page->url, array('abortinstallx' => 1)),
+                new moodle_url($this->page->url, array('abortinstallx' => 1, 'confirmplugincheck' => 0)),
                 get_string('cancelinstallall', 'core_plugin', count($installabortable)),
                 'post',
                 array('class' => 'singlebutton cancelinstallall mr-1')