Deselect failing polars tests (#17362)

Deselect `test_join_4_columns_with_validity` which is failing in nightly CI tests and is reproducible in some systems (xref pola-rs/polars#19870), but apparently not all. Deselect `test_read_web_file` as well that fails on rockylinux8 due to SSL CA issues.

Authors:
  - Peter Andreas Entschev (https://github.com/pentschev)
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

URL: #17362

ci/run_cudf_polars_polars_tests.sh

@@ -13,6 +13,8 @@ DESELECTED_TESTS=(
     "tests/unit/test_cpu_check.py::test_check_cpu_flags_skipped_no_flags" # Mock library error
     "tests/docs/test_user_guide.py" # No dot binary in CI image
     "tests/unit/test_polars_import.py::test_fork_safety" # test started to hang in polars-1.14
+    "tests/unit/operations/test_join.py::test_join_4_columns_with_validity" # fails in some systems, see https://github.com/pola-rs/polars/issues/19870
+    "tests/unit/io/test_csv.py::test_read_web_file" # fails in rockylinux8 due to SSL CA issues
 )
 
 if [[ $(arch) == "aarch64" ]]; then