Customize: Prevent showing SFTP theme installation notification in multisite when user cannot install themes.

Props flixos90.
See #37661, #42184.
Fixes #42674.

Built from https://develop.svn.wordpress.org/trunk@42536


git-svn-id: http://core.svn.wordpress.org/trunk@42365 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-manager.php

@@ -4702,8 +4702,9 @@ final class WP_Customize_Manager {
 				'previewFrameSensitivity' => 2000,
 			),
 			'theme'                  => array(
-				'stylesheet' => $this->get_stylesheet(),
-				'active'     => $this->is_theme_active(),
+				'stylesheet'  => $this->get_stylesheet(),
+				'active'      => $this->is_theme_active(),
+				'_canInstall' => current_user_can( 'install_themes' ),
 			),
 			'url'                    => array(
 				'preview'       => esc_url_raw( $this->get_preview_url() ),