Circle CI Workers are terminated during build

[BobrImperator]

Hi, recently I had an issue with failing build on CircleCI.
It wasn't happening on ember 2.8 and ember-cli-uglify 2.1.0.
Currently we have ember 3.14 and ember-cli-terser 4.0.1

ember build -e=production

Workerpool Worker terminated Unexpectedly
    exitCode: `null`
    signalCode: `SIGKILL`
    workerpool.script: `/home/circleci/project/node_modules/broccoli-terser-sourcemap/lib/worker.js`
    spawnArgs: `/usr/local/bin/node,/home/circleci/project/node_modules/broccoli-terser-sourcemap/lib/worker.js`
    spawnfile: `/usr/local/bin/node`
    stdout: `null`
    stderr: `null`

What I did to fix the issue and make build green again was JOBS=1 ember build -e=production which sets worker pool to 1.
The issue might be related to CircleCi showing available cores of the whole machine rather than the used instance.

Although the concurrency flag logic haven't changed since 2.1.0.

I'm creating this issue mostly for discoverability so others have easier time finding out a possible fix.

Feel free to close this.
I can also add info to this and ember-cli-terser repo's readme.