Limit the Make concurrency to 8 threads

[Atry]

According to the build status, the nightly build continuously fails, while the build succeeded when manually retrying on a devserver.

This might be due to out of memory, and hopefully the issue could be fixed by limiting the concurrency in this PR.

[Atry]

A build on AWS has been triggered:

$ bin/build-on-aws 4.136.0-dev
ONE STATE MACHINE TO RULE THEM ALL
Runs any subset of build steps for any combination of versions + distros.

{
    "executionArn": "arn:aws:states:us-west-2:223121549624:execution:one-state-machine-to-rule-them-all:4.136.0-dev-atry-2021-11-16-15-59",
    "startDate": "2021-11-16T15:59:45.497000-08:00"
}

[Atry]

The version 4.136.0-dev is not supported, launch build-on-aws with a nightly version instead

$ bin/build-on-aws 2021.11.16
ONE STATE MACHINE TO RULE THEM ALL
Runs any subset of build steps for any combination of versions + distros.

{
    "executionArn": "arn:aws:states:us-west-2:223121549624:execution:one-state-machine-to-rule-them-all:2021.11.16-atry-2021-11-16-16-29",
    "startDate": "2021-11-16T16:29:38.368000-08:00"
}

[Atry]

The build status is failed

$ bin/aws-build-status                     
--------------------------------------------------------------------------------------------
|                                      ListExecutions                                      |
+-----------------------------------------------------------------------------+------------+
|                                    name                                     |  status    |
+-----------------------------------------------------------------------------+------------+
|  BuildBinaryPackage_2-fredemmott-2021-11-17-08-16                           |  SUCCEEDED |
|  MakeBinaryPackage_20-fredemmott-2021-11-17-08-17                           |  FAILED    |
|  2021.11.16_PublishBi-fredemmott-2021-11-17-08-14                           |  SUCCEEDED |
|  05915b25-fb67-f189-5ecc-bbd931c682fb_94d064a7-eac1-d754-0470-5f7085daa4f4  |  FAILED    |
|  05c0d668-e2c4-a287-987f-d32d160f02de_42718d3b-24d2-bbd8-7d52-8856d3728e12  |  FAILED    |
|  2021.11.16-atry-2021-11-16-16-29                                           |  FAILED    |
|  901dc55a-7cdd-2397-d748-26407321fa26_e4d84c7d-6880-0ae8-9e9a-3b5a1a6a704e  | :...skipping...
--------------------------------------------------------------------------------------------
|                                      ListExecutions                                      |
+-----------------------------------------------------------------------------+------------+
|                                    name                                     |  status    |
+-----------------------------------------------------------------------------+------------+
|  BuildBinaryPackage_2-fredemmott-2021-11-17-08-16                           |  SUCCEEDED |
|  MakeBinaryPackage_20-fredemmott-2021-11-17-08-17                           |  FAILED    |
|  2021.11.16_PublishBi-fredemmott-2021-11-17-08-14                           |  SUCCEEDED |
|  05915b25-fb67-f189-5ecc-bbd931c682fb_94d064a7-eac1-d754-0470-5f7085daa4f4  |  FAILED    |
|  05c0d668-e2c4-a287-987f-d32d160f02de_42718d3b-24d2-bbd8-7d52-8856d3728e12  |  FAILED    |
|  2021.11.16-atry-2021-11-16-16-29                                           |  FAILED    |
|  901dc55a-7cdd-2397-d748-26407321fa26_e4d84c7d-6880-0ae8-9e9a-3b5a1a6a704e  |  SUCCEEDED |
|  4.136.0-dev-atry-2021-11-16-15-59                                          |  ABORTED   |
|  8913a875-c666-8005-902d-a93fa7bb38dc_8ef9bfe3-34c1-844b-604c-818d73bb5e7c  |  FAILED    |
|  62f313a8-79ed-f7c2-7e38-7585f91ea5a3_d7e68675-3f79-8db7-72b2-28273f608995  |  FAILED    |
+-----------------------------------------------------------------------------+------------+

[fredemmott]

Builds are actually good:

$ curl https://hhvm.com/api/build-status/2021.11.17 --silent | jq
{
  "success": true,
  "version": "2021.11.17",
  "succeeded": [
    "macos-catalina",
    "source",
    "source_gpg",
    "debian-10-buster",
    "debian-11-bullseye",
    "ubuntu-18.04-bionic",
    "ubuntu-20.04-focal",
    "ubuntu-21.04-hirsute"
  ],
  "failed": {},
  "built_not_published": [],
  "not_built": {}
}

[fredemmott]

|  BuildBinaryPackage_2-fredemmott-2021-11-17-08-16                           |  SUCCEEDED |
|  2021.11.16_PublishBi-fredemmott-2021-11-17-08-14                           |  SUCCEEDED |

These are the ones that mattered

[Atry]

There is a failed PR trying to let dune support make's jobserver

ocaml/dune#4331