CI venv jobs should not rely on the hosted runner's CONDA environment variable #26289

Description

The Tests (ubuntu-latest, 3.x, venv) job for #26276 fails in Prepare conda for venv tests before the test suite starts.

The workflow constructs Conda paths from $Env:CONDA, but that environment variable is unset on the runner:

Join-Path: Cannot bind argument to parameter 'Path' because it is null.

Failed job: https://github.com/microsoft/vscode-python/actions/runs/42093715023/job/131577628033

Proposed fix

• Explicitly install/setup Conda for venv test jobs using a commit-pinned action.
• Resolve the Conda executable from PATH and its base directory through conda info --base rather than relying on $Env:CONDA.
• Fail with a clear diagnostic if the Conda executable or base Python cannot be found.
• Verify the venv test matrix on both Ubuntu and Windows, including the working directory containing spaces.

Expected behavior

The workflow records valid condaExecPath and condaPath entries in src/tmp/envPaths.json, and the virtual-environment tests run on both supported operating systems.