Merge bitcoin/bitcoin#31678: ci: Skip read-write of default env vars

fa952ac ci: Skip read-write of default env vars (MarcoFalke)

Pull request description:

  If they remain unset, they use the default anyway. Except for `USER`, but this seems unused anyway.

  Can be checked via:

  ```
  sh-5.2# touch /tmp/empty_env
  sh-5.2# podman run --rm --env-file /tmp/empty_env 'ubuntu:24.04' env
  PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  container=podman
  HOME=/root
  HOSTNAME=19ece5c9e052

ACKs for top commit:
  0xB10C:
    ACK fa952ac
  Prabhat1308:
    utACK [fa952ac](bitcoin/bitcoin@fa952ac)

Tree-SHA512: fe0c173b23cfda3025306303a44ffe32ecc57c2e0e1a2376594696f9887ed22f5105da84e898e790041bf15a4aa42a365fba016710ad269d439dda691977be90

Author: fanquake

ci/test/02_run_container.sh

@@ -7,14 +7,12 @@
 export LC_ALL=C.UTF-8
 export CI_IMAGE_LABEL="bitcoin-ci-test"
 
-set -ex
+set -o errexit -o pipefail -o xtrace
 
 if [ -z "$DANGER_RUN_CI_ON_HOST" ]; then
   # Export all env vars to avoid missing some.
   # Though, exclude those with newlines to avoid parsing problems.
   python3 -c 'import os; [print(f"{key}={value}") for key, value in os.environ.items() if "\n" not in value and "HOME" != key and "PATH" != key and "USER" != key]' | tee "/tmp/env-$USER-$CONTAINER_NAME"
-  # System-dependent env vars must be kept as is. So read them from the container.
-  docker run --platform="${CI_IMAGE_PLATFORM}" --rm "${CI_IMAGE_NAME_TAG}" bash -c "env | grep --extended-regexp '^(HOME|PATH|USER)='" | tee --append "/tmp/env-$USER-$CONTAINER_NAME"
 
   # Env vars during the build can not be changed. For example, a modified
   # $MAKEJOBS is ignored in the build process. Use --cpuset-cpus as an