Remove pip env file; revert pipbuild env changes

[brynpickering]

Reverts changes in pipbuild made during the latest dev release (#681).

I've done this mostly because I get notifications that an action has failed with every PR, but it's just GitHub thinking the environment file is a workflow (since it's a YAML file inside the workflow directory).

Summary of changes in this pull request

• removed pipenv env file
• reverted to list of dependencies in the workflow, with an updated dependency for build (see build -> python-build arup-group/actions-city-modelling-lab#46)

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (e447d17) to head (6cd5d2f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #689   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          26       26           
  Lines        3981     3981           
  Branches      837      768   -69     
=======================================
  Hits         3821     3821           
  Misses         70       70           
  Partials       90       90