Merge pull request #213 from humanmade/fix-linux-uid

Hotfix, use posix_getuid() to get $UID

inc/composer/class-command.php

@@ -379,7 +379,7 @@ protected function exec( InputInterface $input, OutputInterface $output, ?string
 		$has_stdin = ! posix_isatty( STDIN );
 		$has_stdout = ! posix_isatty( STDOUT );
 		if ( $this->is_linux() ) {
-			$user = '$UID';
+			$user = posix_getuid();
 		} else {
 			$user = 'www-data';
 		}