Allow access to extra archive without ca-certificates

ensure we don’t use https to avoid having to install ca-certificates on the
build system, which will skew the "no Internet access" test. The repo is
still signed.

.github/deb-build-docker/Dockerfile

@@ -2,11 +2,14 @@ FROM ubuntu:devel
 
 ENV DEBIAN_FRONTEND=noninteractive
 
-# add our own overlay ppa for dependencies not yet in ubuntu:devel
+# add our own overlay ppa for dependencies not yet in ubuntu:devel.
+# ensure we don’t use https to avoid having to install ca-certificates on the build system, which will
+# skew the "no Internet access" test. The repo is still signed.
 RUN \
    apt-get update && \
    apt-get install -y software-properties-common && \
-   add-apt-repository -y --ppa ppa:ubuntu-wsl-dev/ppa
+   add-apt-repository -y --ppa ppa:ubuntu-wsl-dev/ppa && \
+   sed -i "s#https#http#g" /etc/apt/sources.list.d/*
 
 FROM ubuntu:devel
 COPY --from=0 /etc/apt/sources.list.d/* /etc/apt/sources.list.d/