exp run --run-all: re-enable --jobs concurrency

[PythonFZ]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Currently dvc exp run --run-all --jobs <n> will only start a single celery worker.
This PR would re-enable spawing multiple workers.

I did test this on my machine but I'm not familiar with the dvc test suit so I wasn't able to add a proper unit test for this feature.

Note: the worker id changes from a default 1 to a default 0 with this PR. If this is important the range can be easily adjusted.

[codecov]

Codecov Report

Base: 93.52% // Head: 93.52% // No change to project coverage 👍

Coverage data is based on head (bce0537) compared to base (6fc0a6f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8608   +/-   ##
=======================================
  Coverage   93.52%   93.52%           
=======================================
  Files         457      457           
  Lines       36128    36128           
  Branches     5224     5224           
=======================================
  Hits        33790    33790           
  Misses       1837     1837           
  Partials      501      501           

Impacted Files	Coverage Δ
tests/unit/command/test_queue.py	100.00% <ø> (ø)
dvc/repo/experiments/__init__.py	87.00% <100.00%> (ø)
dvc/repo/experiments/queue/celery.py	85.20% <100.00%> (ø)
tests/unit/repo/experiments/conftest.py	88.67% <100.00%> (ø)

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[pmrowla]

Thanks for the fix @PythonFZ!