fix: edge case where aws config file could be a directory (#890)

I have encountered this issue on github actions

This is weird but I an error with file_get_contents failing.

Author: Sébastien HOUZÉ

src/Credentials/IniFileLoader.php

@@ -48,7 +48,7 @@ public function loadProfiles(array $filepaths): array
                 $homeDir = $homeDir ?? $this->getHomeDir();
                 $filepath = $homeDir . \substr($filepath, 1);
             }
-            if (!\is_readable($filepath)) {
+            if (!\is_readable($filepath) || !\is_file($filepath)) {
                 continue;
             }