buildworker,buildmaster: Use venv for Pip

Debian 12 complains if using Pip without a virtualenv since it could
break the system.

Signed-off-by: Paul Spooren <mail@aparcar.org>

docker/buildmaster/Dockerfile

@@ -46,12 +46,15 @@ RUN \
 		locales \
 		pv \
 		pwgen \
+		python3-venv \
 		python3-pip \
 		signify-openbsd \
 		wget && \
 	apt-get clean && \
 	localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
 
+RUN python3 -m venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
 RUN pip3 install -U pip
 RUN \
 	pip3 install \