atest task loses python interpreter when running with virtualenv under Windows

# For issues
## Steps to reproduce the issue

Under Windows

```msbatch
mkdir atest-python-interpreter
cd atest-python-interpreter
virtualenv -p C:\\Python39\\python.exe venv-py39
venv-py39\\Scripts\\activate
git clone git@github.com:robotframework/SeleniumLibrary.git rf-sl
cd rf-sl
pip install -r requirements-dev.txt
inv atest
```

The last command results in an error
```
(venv-py39) D:\\devsandbox\\atest-python-interpreter\\rf-sl>inv atest
Traceback (most recent call last):
  File "D:\\devsandbox\\atest-python-interpreter\\rf-sl\\utest\\run.py", line 9, in <module>
    from pytest import main as py_main
ModuleNotFoundError: No module named 'pytest'
Not running acceptance test, because unit tests failed.
```
## Error messages and additional information

The last command results in an error
```
(venv-py39) D:\\devsandbox\\atest-python-interpreter\\rf-sl>inv atest
Traceback (most recent call last):
  File "D:\\devsandbox\\atest-python-interpreter\\rf-sl\\utest\\run.py", line 9, in <module>
    from pytest import main as py_main
ModuleNotFoundError: No module named 'pytest'
Not running acceptance test, because unit tests failed.
```

I am running under Windows 10 using Python 3.9.9 within a virtual env

## Expected behavior and actual behavior

I was expecting the runner to use the virtual env I have executed with. But instead it appeas to run `python` from a process not realizing it is a virtualenv that it was started under.

## Environment

Browser: Name and version (Usually available from the about dialogue.)
Browser driver: Name and version
Operating System: Windows 10
Libraries
- Robot Framework: 5.0.1
- Selenium: 4.5.0
- SeleniumLibrary: 6.1.0.dev1
- Interpreter: Python 3.9.9 under a virtualenv

```
(venv-py39) C:\\projects\\SeleniumLibrary\\atest-python-interpreter\\rf-sl>pip list
Package                      Version
---------------------------- -----------
[ ... snip ...]
pytest                       7.1.3
pytest-approvaltests         0.2.4
pytest-mockito               0.0.4
[ ... snip ...]
```

diff --git a/tasks.py b/tasks.py
index ced30c3..fd21efe 100644
--- a/tasks.py
+++ b/tasks.py
@@ -221,7 +221,7 @@ def utest(ctx, suite=None):
     Args:
         suite: Select which suite to run.
     """
-    command = "python utest/run.py"
+    command = f"{sys.executable} utest/run.py"
     if suite:
         command = f"{command} --suite {suite}"
     ctx.run(command)
['tasks.py']