guix: Build depends/qt with our platform definition

Our 'bitcoin-linux-g++' definition better integrates with our depends
system than the stock linux-g++-64 definition.

This fixes a bug whereby Guix builds on x86_64 for x86_64 did not
produce a QMinimalIntegrationPlugin and led to bitcoin-qt not being
built.

contrib/guix/libexec/build.sh

@@ -214,6 +214,7 @@ make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    x86_64_linux_NM=x86_64-linux-gnu-nm \
                                    x86_64_linux_STRIP=x86_64-linux-gnu-strip \
                                    qt_config_opts_i686_linux='-platform linux-g++ -xplatform bitcoin-linux-g++' \
+                                   qt_config_opts_x86_64_linux='-platform linux-g++ -xplatform bitcoin-linux-g++' \
                                    FORCE_USE_SYSTEM_CLANG=1