Update unittests.yml -- manual trigger

[jGaboardi]

• This PR add a manual trigger for unittests.yml
• xref Version 1.5 #136 (comment)

[jGaboardi]

After merging this PR you will see like this on [https://github.com/pysal/spreg/actions/workflows/unittests.yml]:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.1%. Comparing base (5ec4601) to head (1606feb).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #137   +/-   ##
=====================================
  Coverage   69.1%   69.1%           
=====================================
  Files         40      40           
  Lines       8727    8738   +11     
  Branches    1253    1253           
=====================================
+ Hits        6028    6037    +9     
- Misses      2319    2321    +2     
  Partials     380     380           

see 8 files with indirect coverage changes

[pedrovma]

Awesome, thanks!