Pulumi using wrong Python/Python path in pipx install

[JimMadge]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates.
• [x] This is a problem observed when deploying a Data Safe Haven.
• [x] I can reproduce this with the latest version.
• [x] I have read through the documentation.
• [x] This isn't an open-ended question (open a discussion if it is).

:computer: System information

• Operating System:
• Data Safe Haven version:

:package: Packages

List of packages

```none Paste list of packages here ```

:no_entry_sign: Describe the problem

After pipx install, when calling dsh Pulumi will look for a Python interpreter and find system Python, not the Python venv with all of the dependencies. It will fail because of missing packages.

On option to fix, can declare venv path in Pulumi package settings. We set these here

:deciduous_tree: Log messages

Relevant log messages

```none Your log details here ```

:recycle: To reproduce

[craddm]

There is an environment variable we can set to tell Pulumi to use a specific python version: PULUMI_PYTHON_CMD - see https://www.pulumi.com/docs/iac/cli/environment-variables/