correct mistake on downloaded executable for cvc4, make it the default

smtcoq+cvc4-container/Dockerfile

@@ -6,6 +6,9 @@ USER root
 RUN make install
 WORKDIR /usr/local/bin
 RUN wget http://cvc4.cs.stanford.edu/downloads/builds/x86_64-linux-opt/cvc4-1.5-x86_64-linux-opt
+RUN chmod a+x cvc4-1.5-x86_64-linux-opt
+RUN mv cvc4 cvc4_compiled_from_sources
+RUN ln cvc4-1.5-x86_64-linux-opt cvc4
 USER johndoe
 WORKDIR /home/johndoe
 ENV LFSCSIGS=/home/johndoe/smtcoq/src/lfsc/tests/signatures/