Explicitly add unzip as a dependency

The `unzip` package is not installed by default in Ubuntu. It's currently pulled in as an implicit dependency of the Bazel package, but that breaks if Bazel ever removes that dependency (or when changing the Dockerfile to not install Bazel).

build/bazelbuilder/Dockerfile

@@ -14,6 +14,7 @@ RUN apt-get update \
     gnupg2 \
     libncurses-dev \
     make \
+    unzip \
  && update-alternatives --install /usr/bin/clang clang /usr/bin/clang-10 100 \
     --slave /usr/bin/clang++ clang++ /usr/bin/clang++-10 \
  && apt-get clean