build: address breakage by golang:1.19 Docker image switching to bookworm

Debian bookworm apparently _requires_ pip to be used in venv, at least
when we use it inside the build.Dockerfile.

So, do that.

build.Dockerfile

@@ -2,12 +2,18 @@ FROM !SUBSTITUTED_BY_MAKEFILE
 
 RUN apt-get update && apt-get install -y \
     python3-pip \
+    python3-venv \
     unzip \
     gawk
 
 ADD build.installprotoc.bash ./
 RUN bash build.installprotoc.bash
 
+# setup venv
+ENV VIRTUAL_ENV=/opt/venv
+RUN python3 -m venv $VIRTUAL_ENV
+ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+
 ADD lazy.sh /tmp/lazy.sh
 ADD docs/requirements.txt /tmp/requirements.txt
 ENV ZREPL_LAZY_DOCS_REQPATH=/tmp/requirements.txt