Fix adding build-compiler-rt.sh after wiping the build dir

With newer docker versions, "COPY foo .", when the workdir doesn't exist,
produces a plain file named as the workdir, breaking the rest of the build.

Dockerfile

@@ -44,7 +44,7 @@ RUN ./build-libcxx.sh $TOOLCHAIN_PREFIX && \
     rm -rf /build
 
 # Build sanitizers
-COPY build-compiler-rt.sh .
+COPY build-compiler-rt.sh ./
 RUN ./build-compiler-rt.sh $TOOLCHAIN_PREFIX --build-sanitizers && \
     rm -rf /build