Just output a warning but don't prevent usage through non-cli SAPIs

bin/composer

@@ -2,8 +2,7 @@
 <?php
 
 if (PHP_SAPI !== 'cli') {
-    echo 'Composer should be invoked via the CLI version of PHP, not the '.PHP_SAPI.' SAPI'.PHP_EOL;
-    exit(1);
+    echo 'Warning: Composer should be invoked via the CLI version of PHP, not the '.PHP_SAPI.' SAPI'.PHP_EOL;
 }
 
 require __DIR__.'/../src/bootstrap.php';