Blender Git Loki

Tests: use environment variable for leak sanitizer

Don't override other `LSAN_OPTIONS` like suppression file set in
the environment variable.

Old code added in {rB38ff5064b33ccb8} and {rB5f4e99b7a2b8376}

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D9207