TST: run conda test from installed version

[ZLLentz]

For future discussion: this makes my pcdswidgets tests pass

• I've added a build step to pcdswidgets
• This builds and installs correctly
• Without this change, the conda tests run on the pre-built version of the package

Things to consider:

• This makes the test output pretty awful (micromamba/conda-bld/pcdswidgets_1773337587302/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_pla/lib/python3.12/site-packages/pcdswidgets/tests/test_builder.py::test_it_was_built[ui_source0] PASSED [ 1%])
• This ensures that we're testing on the version of the package that gets installed
• I didn't change the pypi tests because the pypi build runs and builds install-time files in the cloned repo, so the tests work for me, but maybe this should also be done on the pypi tests.