Set pythonInterpreter in set-python-manylinux-variables-step.yml (#19105

)

### Description
Set pythonInterpreter in set-python-manylinux-variables-step.yml. To fix
a build error:

```
Starting: Set Python manylinux variables
==============================================================================
Task         : Python script
Description  : Run a Python file or inline script
Version      : 0.231.1
Author       : Microsoft Corporation
Help         : https://docs.microsoft.com/azure/devops/pipelines/tasks/utility/python-script
==============================================================================
##[error]Parameter 'toolPath' cannot be null or empty.
Finishing: Set Python manylinux variables
```
The error was because today I deleted a bunch of software from the VM
image. The task might fail if no Python versions are found in
$(Agent.ToolsDirectory).

tools/ci_build/github/azure-pipelines/templates/set-python-manylinux-variables-step.yml

@@ -5,6 +5,7 @@ steps:
 - task: PythonScript@0
   displayName: 'Set Python manylinux variables'
   inputs:
+    pythonInterpreter: /usr/bin/python3
     scriptSource: inline
     script: |
       version = "$(PythonVersion)"