Circle CI Workers are terminated during build

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.