fix ci after change in nightly

-Zgcc-lld was stabilized in
rust-lang/compiler-team#510 under a
different name.

Dockerfile

@@ -26,7 +26,7 @@ RUN source $HOME/.cargo/env && \
         GCC_VERSION=$($MUSL_TARGET-gcc --version | grep gcc | awk '{print $3}') && \
         echo -e "\
 [build]\n\
-rustflags = [\"-L\", \"native=/$MUSL_TARGET-cross/$MUSL_TARGET/lib\", \"-L\", \"native=/$MUSL_TARGET-cross/lib/gcc/$MUSL_TARGET/$GCC_VERSION/\", \"-l\", \"static=gcc\", \"-Z\", \"gcc-ld=lld\"]\n\
+rustflags = [\"-L\", \"native=/$MUSL_TARGET-cross/$MUSL_TARGET/lib\", \"-L\", \"native=/$MUSL_TARGET-cross/lib/gcc/$MUSL_TARGET/$GCC_VERSION/\", \"-l\", \"static=gcc\", \"-Z\", \"-Clink-self-contained=linker\", \"-Clinker-flavor=gcc-lld\"]\n\
 [target.$RUST_TARGET]\n\
 linker = \"$MUSL_TARGET-gcc\"\n\
 [unstable]\n\