LLVM symbolizer not present in PATH during multiversion suites on ASan and UBSan variants

We set the path_prefix expansion for the burn_in_tests_WT, multiversion, and sharding_last_stable_mongos_and_mixed_shards tasks in order to add the symlinks for earlier versions of MongoDB to the PATH environment variable. This ends up overriding the path_prefix expansion set at the variant-level for ubuntu1604-debug-asan, ubuntu1410-asan, and ubuntu1604-debug-ubsan, resulting in no symbolization when any of those tasks fail on these variants.