Limit concurrency for nightly jobs

[akihironitta]

This PR limits the number of concurrent nightly jobs to let other repos to use the resource of GitHub-hosted runners. Currently, we have 200 scheduled nightly jobs, and each takes from 3.5 min to 15 min, occupying the max number of concurrent jobs (i.e., 20) across all repos pyg-team org:

This should be ok because nightly builds don't need to be avaialble ASAP.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.58%. Comparing base (fb0b5df) to head (2feb332).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
+ Coverage   86.40%   86.58%   +0.18%     
==========================================
  Files          35       35              
  Lines        1214     1208       -6     
==========================================
- Hits         1049     1046       -3     
+ Misses        165      162       -3     

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