Fix nginx to run test test_downloadp.y local as non-root user

[tduf]

Logs

$ ./test/wait-for-hawkbit-online && dbus-run-session -- pytest -v -o log_cli=true test/test_download.py
[...]
test/test_download.py::test_download_too_slow
-------------------------------------------------------- live log call --------------------------------------------------------
INFO nginx running: nginx -c /tmp/pytest-of-thibaud/pytest-8/nginx0/nginx.conf -p .
INFO nginx nginx: [alert] could not open error log file: open() "/var/log/nginx/error.log" failed (13: Permission denied)
INFO nginx 2024/06/28 15:41:56 [emerg] 1450257#1450257: mkdir() "/var/lib/nginx/tmp/client_body" failed (13: Permission denied)
SKIPPED (nginx failed, use -s to see logs)                                                                              [ 42%]
test/test_download.py::test_download_partials_without_resume
------------------------------------------------------- live log setup --------------------------------------------------------
INFO nginx running: nginx -c /tmp/pytest-of-thibaud/pytest-8/nginx1/nginx.conf -p .
INFO nginx nginx: [alert] could not open error log file: open() "/var/log/nginx/error.log" failed (13: Permission denied)
INFO nginx 2024/06/28 15:41:56 [emerg] 1450258#1450258: mkdir() "/var/lib/nginx/tmp/client_body" failed (13: Permission denied)
SKIPPED (nginx failed, use -s to see logs)                                                                              [ 57%]
test/test_download.py::test_download_partials_with_resume SKIPPED (nginx failed, use -s to see logs)                    [ 71%]
test/test_download.py::test_download_slow_with_resume
-------------------------------------------------------- live log call --------------------------------------------------------
INFO nginx running: nginx -c /tmp/pytest-of-thibaud/pytest-8/nginx2/nginx.conf -p .
INFO nginx nginx: [alert] could not open error log file: open() "/var/log/nginx/error.log" failed (13: Permission denied)
INFO nginx 2024/06/28 15:41:56 [emerg] 1450259#1450259: mkdir() "/var/lib/nginx/tmp/client_body" failed (13: Permission denied)
SKIPPED (nginx failed, use -s to see logs)

Run on Fedora 40 with following rpm packages installed:

meson
ninja-build
libcurl-devel
json-glib-devel
python3-sphinx

# Needed by pip install -r test-requirements.txt
pkg-config
cairo
cairo-devel
cairo-gobject-devel
gobject-introspection
gobject-introspection-devel

nginx
nginx-all-modules

[tduf]

With this fix, there is still issues with test/test_download.py::test_download_partials_without_resume (download succeed) and test_download_partials_with_resume (download succeed without resume).