Set TEST_NON_ROOT_USER in CI

[arnaud-lb]

Some times ago I added a few opcache.preload_user tests that need a TEST_NON_ROOT_USER variable, but I never defined this variable in CI (these tests also need to run as root, but there were no CI job that ran as root at the time).

Here I fix this by setting the variable in the ALPINE nightly job, which runs as root.

Also set ASAN_OPTIONS=detect_leaks=0 for these tests, because the preload child process doesn't exit cleanly:

php-src/ext/opcache/ZendAccelerator.c

Line 5059 in 6141594

exit(ret == SUCCESS ? 0 : 1);

Targeting 8.4 because .github/actions/test-alpine does not exist in earlier branches.