qEnvironmentVariable: avoid a temporary QByteArray

Because we don't need one. I guess I was just lazy.

Amends b12fd1fa9d ("Long live
qEnvironmentVariable()").

Change-Id: I5b9141c5a32f9d784e0bfffdc3e4d742790aebdd
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit ce95c26034cc5b3ae8094c1521221e5b2f13ecfa)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 36d260014fe28be17259abee7547b660669dfdc4)

src/corelib/global/qtenvironmentvariables.cpp

@@ -126,8 +126,9 @@ QString qEnvironmentVariable(const char *varName, const QString &defaultValue)
     buffer.chop(1);
     return buffer;
 #else
-    QByteArray value = qgetenv(varName);
-    if (value.isNull())
+    const auto locker = qt_scoped_lock(environmentMutex);
+    const char *value = ::getenv(varName);
+    if (!value)
         return defaultValue;
 // duplicated in qfile.h (QFile::decodeName)
 #if defined(Q_OS_DARWIN)