[vcpkg] Add a definition for OVERLAY_TRIPLET_ENV (microsoft#14436)

This definition is needed in C++14/gcc 6 to compile.

src/vcpkg/vcpkgcmdarguments.cpp

@@ -912,6 +912,7 @@ namespace vcpkg
     constexpr StringLiteral VcpkgCmdArguments::OVERLAY_PORTS_ENV;
     constexpr StringLiteral VcpkgCmdArguments::OVERLAY_PORTS_ARG;
     constexpr StringLiteral VcpkgCmdArguments::OVERLAY_TRIPLETS_ARG;
+    constexpr StringLiteral VcpkgCmdArguments::OVERLAY_TRIPLETS_ENV;
 
     constexpr StringLiteral VcpkgCmdArguments::BINARY_SOURCES_ARG;