Facilitate guix building with CROSS_LIBRARY_PATH

See discussion in https://github.com/bitcoin/bitcoin/pull/15277.

--- a/qtbase/mkspecs/features/toolchain.prf
+++ b/qtbase/mkspecs/features/toolchain.prf
@@ -231,8 +231,8 @@ isEmpty($${target_prefix}.INCDIRS) {
             add_libraries = false
             for (line, output) {
                 line ~= s/^[ \\t]*//  # remove leading spaces
-                contains(line, "LIBRARY_PATH=.*") {
-                    line ~= s/^LIBRARY_PATH=//  # remove leading LIBRARY_PATH=
+                contains(line, "(CROSS_)?LIBRARY_PATH=.*") {
+                    line ~= s/^(CROSS_)?LIBRARY_PATH=//  # remove leading (CROSS_)?LIBRARY_PATH=
                     equals(QMAKE_HOST.os, Windows): \
                         paths = $$split(line, ;)
                     else: \