fix: read property to determine if using python virtual env (#65)

## Description

In addition to reading environment variable `EXHORT_PYTHON_VIRTUAL_ENV`
to determining if using python virtual environment, also ready system
property.

## Checklist

- [x] I have followed this repository's contributing guidelines.
- [x] I will adhere to the project's code of conduct.

## Additional information

> Anything else?

src/main/java/com/redhat/exhort/providers/PythonPipProvider.java

@@ -189,7 +189,9 @@ private PythonControllerBase getPythonController() {
     String[] parts = pythonPipBinaries.split(";;");
     var python = parts[0];
     var pip = parts[1];
-    String useVirtualPythonEnv = Objects.requireNonNullElse(System.getenv("EXHORT_PYTHON_VIRTUAL_ENV"), "false");
+    String useVirtualPythonEnv = Objects.requireNonNullElseGet(
+      System.getenv("EXHORT_PYTHON_VIRTUAL_ENV"),
+      () -> Objects.requireNonNullElse(System.getProperty("EXHORT_PYTHON_VIRTUAL_ENV"), "false"));
     PythonControllerBase pythonController;
     if(this.pythonController == null) {
       if (Boolean.parseBoolean(useVirtualPythonEnv)) {