Use platform-specific activation, pin setuptools 69.2.0 (#36454)

* pin setuptools to 69.2.0
* use virtualenv for our virtual environment creation instead of default venv
* source the virtual environment just before build steps, versus prepending path which was working unreliably
* remove venv use from test phase, as it wasn't actually having the effect we originally expected

Author: scbedd

eng/pipelines/templates/steps/build-test.yml

@@ -31,8 +31,6 @@ steps:
     parameters:
       versionSpec: '${{ parameters.PythonVersion }}'
 
-  - template: /eng/pipelines/templates/steps/use-venv.yml
-
   - template: set-dev-build.yml
     parameters:
       ServiceDirectory: ${{ parameters.ServiceDirectory }}