McuSupport: chop RTOS environment variable suffix

Change-Id: Iba35246af403a00bd0f750e3769e22a5010825eb
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/mcusupport/mcusupportoptions.h

@@ -51,8 +51,9 @@ namespace Internal {
 
 class McuAbstractPackage;
 
-struct McuSdkRepository
+class McuSdkRepository
 {
+public:
     Packages packages;
     Targets mcuTargets;
     void deletePackagesAndTargets();