guix: Fix "Ignoring empty string" CMake warning for non-Linux hosts

Using shell parameter expansion ensures that on non-Linux hosts the
build script won't pass an empty string argument to CMake. This resolves
the "Ignoring empty string" warning.

contrib/guix/libexec/build.sh

@@ -166,7 +166,7 @@ mkdir -p "$DISTSRC"
           -DWITH_CCACHE=OFF \
           -Werror=dev \
           ${CONFIGFLAGS} \
-          "${CMAKE_EXE_LINKER_FLAGS}"
+          ${CMAKE_EXE_LINKER_FLAGS+"$CMAKE_EXE_LINKER_FLAGS"}
 
     # Build Bitcoin Core
     cmake --build build -j "$JOBS"