Pin the Python 3 version to 3.8 in Clang/GCC Linux Containers (#3147)

This affects only clang and gcc linux images used to build Cobalt on
older toolchains to monitor for breaking changes to partner toolchains.
The python version available in Ubuntu Bionic is 3.6.9 which is older
than what we guarantee at 3.8.x. We cannot upgrade the Linux distro here
since it provides the specific clang 3.9 and gcc 6.3 versions, so as a
workaround we backport python 3.8 from a PPA.

b/338466640

docker/linux/clang-3-9/Dockerfile

@@ -24,7 +24,15 @@ RUN apt update -qqy \
     && add-apt-repository -y ppa:savoury1/backports \
     && apt update -qqy \
     && apt install -qqy --no-install-recommends git nasm \
+        python3.8 python3.8-dev python3.8-venv \
     && /opt/clean-after-apt.sh
 
+# TODO(b/338650174): Upgrade the base linux distro to avoid backporting python
+# 3.8 manually. Additionally, this involves upgrading clang or backporting
+# the current version (3.9) to the new distro.
+RUN python3.8 -m venv /python38_env
+ENV VIRTUAL_ENV /python38_env
+ENV PATH /python38_env/bin:$PATH
+
 CMD gn gen ${OUTDIR}/${PLATFORM}_${CONFIG} --args="target_platform=\"${PLATFORM}\" build_type=\"${CONFIG}\" using_old_compiler=true build_with_separate_cobalt_toolchain=true" && \
     ninja -v -j ${NINJA_PARALLEL} -C ${OUTDIR}/${PLATFORM}_${CONFIG} ${TARGET:-cobalt_install}

docker/linux/gcc-6-3/Dockerfile

@@ -24,7 +24,15 @@ RUN apt update -qqy \
     && add-apt-repository -y ppa:savoury1/backports \
     && apt update -qqy \
     && apt install -qqy --no-install-recommends git nasm \
+        python3.8 python3.8-dev python3.8-venv \
     && /opt/clean-after-apt.sh
 
+# TODO(b/338650174): Upgrade the base linux distro to avoid backporting python
+# 3.8 manually. Additionally, this involves upgrading gcc or backporting
+# the current version (6.3) to the new distro.
+RUN python3.8 -m venv /python38_env
+ENV VIRTUAL_ENV /python38_env
+ENV PATH /python38_env/bin:$PATH
+
 CMD gn gen ${OUTDIR}/${PLATFORM}${SB_API_VERSION:+-sbversion-$SB_API_VERSION}_${CONFIG:-debug} --args="target_platform=\"${PLATFORM}\" build_type=\"${CONFIG:-debug}\" build_with_separate_cobalt_toolchain=true using_old_compiler=true ${SB_API_VERSION:+sb_api_version=$SB_API_VERSION}" && \
     ninja -v -j ${NINJA_PARALLEL} -C ${OUTDIR}/${PLATFORM}${SB_API_VERSION:+-sbversion-$SB_API_VERSION}_${CONFIG:-debug} ${TARGET:-cobalt_install}