travis: dont hang when installing tzdata

Author: nunoplopes

contrib/ci/Dockerfiles/z3_base_ubuntu_20.04.Dockerfile

@@ -1,5 +1,6 @@
 FROM ubuntu:20.04
 
+ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && \
     apt-get -y --no-install-recommends install \
         cmake \
@@ -14,6 +15,7 @@ RUN apt-get update && \
         graphviz \
         python3 \
         python3-setuptools \
+        python-is-python3 \
         sudo
 
 RUN curl -SL https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb --output packages-microsoft-prod.deb && \

contrib/ci/scripts/sanitizer_env.sh

@@ -23,7 +23,7 @@ if [ "X${ASAN_BUILD}" = "X1" ]; then
     export ASAN_OPTIONS="${ASAN_OPTIONS},print_suppressions=0"
   fi
 
-  : ${ASAN_SYMBOLIZER_PATH?"ASAN_SYMBOLIZER_PATH must be specified"}
+  #: ${ASAN_SYMBOLIZER_PATH?"ASAN_SYMBOLIZER_PATH must be specified"}
 
   # Run command without checking for leaks
   function run_no_lsan() {