globals.hh: don't use '==' to compare string literals

Saw this in logs, also reported here:

NixOS@8e6108f#r28707288

Author: dtzWill

src/libstore/globals.hh

@@ -296,7 +296,7 @@ public:
         "listed in 'trusted-public-keys'."};
 
     Setting<StringSet> extraPlatforms{this,
-        SYSTEM == "x86_64-linux" ? StringSet{"i686-linux"} : StringSet{},
+        std::string{SYSTEM} == "x86_64-linux" ? StringSet{"i686-linux"} : StringSet{},
         "extra-platforms",
         "Additional platforms that can be built on the local system. "
         "These may be supported natively (e.g. armv7 on some aarch64 CPUs "