Change runner from true to echo

Looks like a recent cargo regression is causing issues here!

Author: alexcrichton

ci/docker/x86_64-pc-windows-gnu/Dockerfile

@@ -6,5 +6,5 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   gcc-mingw-w64-x86-64
 
 # No need to run tests, we're just testing that it compiles
-ENV CARGO_TARGET_X86_64_PC_WINDOWS_GNU_RUNNER=true \
+ENV CARGO_TARGET_X86_64_PC_WINDOWS_GNU_RUNNER=echo \
     CARGO_TARGET_X86_64_PC_WINDOWS_GNU_LINKER=x86_64-w64-mingw32-gcc