OCAML_RUNTIME_EVENTS_PRESERVE doesn't work alongside olly

[curche]

Steps to reproduce

1. Obtain olly: either through opam or from source using dune build
2. Try running command while setting the environment variable that keeps .events file. For eg:
   OCAML_RUNTIME_EVENTS_PRESERVE=1 dune exec olly -- gc-stats --dir=./tmp-tmp "../infer/infer/bin/infer analyze --no-progress-bar --no-report --multicore -j $(getconf _NPROCESSORS_ONLN)"

Expected behavior

I was hoping that the .events file weren't deleted at the end of the run

Actual behavior

No .events file are available at the end of the run

Other details

Setting all the runtime events environment variables does seem to work
OCAMLRUNPARAM=e=50 OCAML_RUNTIME_EVENTS_PRESERVE=1 OCAML_RUNTIME_EVENTS_DIR=./tmp-tmp OCAML_RUNTIME_EVENTS_START=1 dune exec olly -- gc-stats --dir=./tmp-tmp "../infer/infer/bin/infer analyze --no-progress-bar --no-report --multicore -j $(getconf _NPROCESSORS_ONLN)"

Looking at the source, this is related to olly setting these variables and removing the files during cleanup. The runtime saving these files likely happens after olly exits and so there are two .events file available after running the latter command. I'm unsure what might be the best behavior in this case

[tmcgilchrist]

The code should honour OCAML_RUNTIME_EVENTS_PRESERVE=1 if it's passed in via the olly environment. Happy to review a PR implementing this.