Fix pytest.skipif condition in installation test

Updated the `skipif` condition to always skip the test unless run explicitly. This ensures the installation test isn't executed unintentionally during standard test runs.

Author: Leguark

tests/test_installation.py

@@ -1,7 +1,7 @@
 import pytest
 
 
-@pytest.mark.skipif("Run explicitly to test installation")
+@pytest.mark.skipif(True, "Run explicitly to test installation")
 def test_pyvista():
     import pyvista