`csmock` works with `systemd-nspawn` only on RHEL-9 and newer

[kdudka]

csmock does not work well with systemd-nspawn on RHEL-8 because systemd-239-78.e8 does not include a systemd hack which mock uses internally and which csmock relies on. If the environment variable is ignored by systemd-nspawn, /tmp is over-mounted by a fresh tmpfs file system on each invocation of mock --chroot or mock --shell, and whatever we put into /tmp is missing in the next steps. For example:

>>> 2023-11-30 14:42:13 "tar -cP '/usr/share/csmock/scripts' '/tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1.src.rpm' '/tmp/csmockpvtw2ssw/gitleaks' | '/usr/bin/mock' '-r' 'rhel-8.7.0.z-x86_64' '--plugin-option=tmpfs:keep_mounted=True' '--config-opts=print_main_output=True' '--quiet' '--shell' 'tar -xC/'"
tar: Removing leading `/' from member names

>>> 2023-11-30 14:42:15 "/usr/bin/mock" "-r" "rhel-8.7.0.z-x86_64" "--plugin-option=tmpfs:keep_mounted=True" "--config-opts=print_main_output=True" "--quiet" "--chroot" "rpm -Uvh --nodeps '/tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1.src.rpm'"
error: open of /tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1.src.rpm failed: No such file or directory

[...]

>>> 2023-11-30 14:42:30 "/usr/bin/mock" "-r" "rhel-8.7.0.z-x86_64" "--plugin-option=tmpfs:keep_mounted=True" "--config-opts=print_main_output=True" "--quiet" "--chroot" "/tmp/csmockpvtw2ssw/gitleaks detect --no-git cmd --source=/builddir/build/BUILD --report-path=/builddir/gitleaks-capture.sarif --report-format=sarif 2>/builddir/gitleaks-capture.log"

>>> 2023-11-30 14:42:32 "'/usr/bin/mock' '-r' 'rhel-8.7.0.z-x86_64' '--plugin-option=tmpfs:keep_mounted=True' '--config-opts=print_main_output=True' '--quiet' '--shell' 'tar -c --remove-files '/builddir/gitleaks-capture.sarif' '/builddir/gitleaks-capture.log'' | tar -xC '/tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1/debug/raw-results'"
tar: Removing leading `/' from member names
tar: /builddir/gitleaks-capture.sarif: Cannot stat: No such file or directory
tar: Removing leading `/' from hard link targets
tar: Exiting with failure status due to previous errors

>>> 2023-11-30 14:42:34 "csgrep '/tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1/debug/raw-results/builddir/gitleaks-capture.sarif' --mode=json --warning-rate-limit=1024 --limit-msg-len=512 > '/tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1/debug/uni-results/gitleaks-capture.js'"
/tmp/csmockpvtw2ssw/cloud-init-21.1-15.el8_6.1/debug/raw-results/builddir/gitleaks-capture.sarif: failed to open input file

!!! 2023-11-30 14:42:34 error: post-process hook failed