Fix inconsistency with .env loaders out of the box

PHP .env loaders such as `symfony/dotenv` or `vlucas/phpdotenv` populate `$_SERVER` and `$_ENV` out of the box, but don't call `putenv()` in order to preserve thread safeness. This means .env is loaded by them, but AWS SDK does not read it, creating inconsistent experience when handling real env variables and env variables loaded by these loaders

src/functions.php

@@ -608,6 +608,6 @@ function getenv(string $name)
 {
     $out = \getenv($name);
 
-    return $out === false ? ($_SERVER[$name] ?? false) : $out;
+    return $out === false ? ($_ENV[$name] ?? false) : $out;
 }
 

tests/FunctionsTest.php

@@ -523,6 +523,6 @@ public function getIniFileServiceTestCases()
     public function testgetenv()
     {
         $this->assertEquals(false, Aws\getenv('FOO'));
-        $this->assertEquals($_SERVER['FOO'] = 'bar', Aws\getenv('FOO'));
+        $this->assertEquals($_ENV['FOO'] = 'bar', Aws\getenv('FOO'));
     }
 }