ci/lava: Use per-job rootfs overlay for environment

Trying to get arbitrary strings suitably quoted for shell, embedded in a
YAML file, processed by Python templating, is like seven bad ideas all
embedded into one big can of bees.

Reuse the same script we use for bare-metal to generate the environment,
tar that up into a per-job overlay which is added to the
inter-pipeline-reusable rootfs built by the container jobs and the
intra-pipeline-reusable overlay built by the build jobs.

@anholt wrote a chunk of this - replacing the $ENV_VARS GitLab CI
variable with a Python loop across the POSIX job environment - in
!11192, but this still had YAML quoting nightmares, and was more
needless duplication between LAVA and bare-metal.

The diff is large and annoying, but is mostly a sed job to get
ENV_VARS="FOO=bar BAZ=quux" into FOO: bar\nBAZ: quux.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Co-authored-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11309>

.gitlab-ci/prepare-artifacts.sh

@@ -45,6 +45,7 @@ find . -path \*/ci/\*.txt \
 # packed separately in the zip file.
 mkdir -p artifacts/
 tar -cf artifacts/install.tar install
+cp -Rp .gitlab-ci/common artifacts/ci-common
 cp -Rp .gitlab-ci/lava artifacts/
 
 if [ -n "$MINIO_ARTIFACT_NAME" ]; then