Allow specifying the job id that the results should be associated with

[IanButterworth]

Closes #83

I found that this pre-exit hook isn't picking up additions to BUILDKITE_ENV_FILE so I added a source here.

I also was hitting an issue where sourcing BUILDKITE_ENV_FILE threw unbound variable errors like below because of scripts stored in env vars. I assume they're not escaped enough? Perhaps thats a bug?

...
BUILDKITE_SCRIPT_PATH="echo \"--- Download test results from test job\"\n# Debug: Verify env file contents\ncat \"$BUILDKITE_ENV_FILE\"\n\necho \"BUILDKITE_PLUGIN_TEST_COLLECTOR_JOB_ID=$BUILDKITE_PLUGIN_TEST_COLLECTOR_JOB_ID\"\nbuildkite-agent artifact download --step \"test_aarch64-apple-darwin\" \"results*.json\" . \|\| true\n"
BUILDKITE_BUILD_AUTHOR="Ian Butterworth"
...

caused

/tmp/job-env-019573a5-4a0a-47d5-8a51-c0ad52c802c63733389565: line 12: BUILDKITE_PLUGIN_TEST_COLLECTOR_JOB_ID: unbound variable

I think the same would happen with BUILDKITE_COMMAND.

Hence why I added set +u around the source.

[IanButterworth]

I opted to not source the env var file because of buildkite/agent#3227

This needs a way for steps earlier than when this runs to set job_id. Currently I don't know a way.