Reapply the change to allow to set the composer binary path

This change was somehow changed back during the renaming of the file I think. So this is just restoring the change.
symfony · nicolas-grekas · Jun 22, 2021 · Jun 22, 2021 · Jun 22, 2021 · 9589b52a78bbb21602043511209f46a37e9fc886
commit 9589b52a78bbb21602043511209f46a37e9fc886
diff --git a/src/Symfony/Bridge/PhpUnit/bin/simple-phpunit.php b/src/Symfony/Bridge/PhpUnit/bin/simple-phpunit.php
@@ -150,12 +150,14 @@
    putenv('SYMFONY_DEPRECATIONS_HELPER=disabled');
 }

-$COMPOSER = file_exists($COMPOSER = $oldPwd.'/composer.phar')
-    || ($COMPOSER = rtrim((string) ('\\' === \DIRECTORY_SEPARATOR ? preg_replace('/[\r\n].*/', '', `where.exe composer.phar 2> NUL`) : `which composer.phar 2> /dev/null`)))
-    || ($COMPOSER = rtrim((string) ('\\' === \DIRECTORY_SEPARATOR ? preg_replace('/[\r\n].*/', '', `where.exe composer 2> NUL`) : `which composer 2> /dev/null`)))
-    || file_exists($COMPOSER = rtrim((string) ('\\' === \DIRECTORY_SEPARATOR ? `git rev-parse --show-toplevel 2> NUL` : `git rev-parse --show-toplevel 2> /dev/null`)).\DIRECTORY_SEPARATOR.'composer.phar')
-    ? ('#!/usr/bin/env php' === file_get_contents($COMPOSER, false, null, 0, 18) ? $PHP : '').' '.escapeshellarg($COMPOSER) // detect shell wrappers by looking at the shebang
-    : 'composer';
+if (false === $COMPOSER = getenv('COMPOSER_BINARY')) {
+    $COMPOSER = file_exists($COMPOSER = $oldPwd.'/composer.phar')
+        || ($COMPOSER = rtrim((string) ('\\' === \DIRECTORY_SEPARATOR ? preg_replace('/[\r\n].*/', '', `where.exe composer.phar 2> NUL`) : `which composer.phar 2> /dev/null`)))
+        || ($COMPOSER = rtrim((string) ('\\' === \DIRECTORY_SEPARATOR ? preg_replace('/[\r\n].*/', '', `where.exe composer 2> NUL`) : `which composer 2> /dev/null`)))
+        || file_exists($COMPOSER = rtrim((string) ('\\' === \DIRECTORY_SEPARATOR ? `git rev-parse --show-toplevel 2> NUL` : `git rev-parse --show-toplevel 2> /dev/null`)).\DIRECTORY_SEPARATOR.'composer.phar')
+        ? ('#!/usr/bin/env php' === file_get_contents($COMPOSER, false, null, 0, 18) ? $PHP : '').' '.escapeshellarg($COMPOSER) // detect shell wrappers by looking at the shebang
+        : 'composer';
+}

 $prevCacheDir = getenv('COMPOSER_CACHE_DIR');
 if ($prevCacheDir) {