set njobs for default algorithm

[freddyaboulton]

Pull Request Description

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After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov]

Codecov Report

Merging #3548 (6ceea41) into main (1e4d496) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3548     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        335     335             
  Lines      33238   33252     +14     
=======================================
+ Hits       33109   33123     +14     
  Misses       129     129             

Impacted Files	Coverage Δ
...valml/automl/automl_algorithm/default_algorithm.py	100.0% <ø> (ø)
evalml/automl/automl_search.py	99.6% <ø> (ø)
evalml/automl/automl_algorithm/automl_algorithm.py	100.0% <100.0%> (ø)
evalml/tests/automl_tests/test_automl.py	99.5% <100.0%> (+0.1%)	⬆️

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[jeremyliweishih]

Much appreciated 🙏