guix: ignore additional failing certvalidator test

Similar to 8588591.

```bash
ERROR: test_revocation_mode_soft (tests.test_validate.ValidateTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/guix-build-python-certvalidator-0.1-1.a145bf2.drv-0/source/tests/test_validate.py", line 85, in test_revocation_mode_soft
    validate_path(context, path)
  File "/tmp/guix-build-python-certvalidator-0.1-1.a145bf2.drv-0/source/tests/../certvalidator/validate.py", line 50, in validate_path
    return _validate_path(validation_context, path)
  File "/tmp/guix-build-python-certvalidator-0.1-1.a145bf2.drv-0/source/tests/../certvalidator/validate.py", line 358, in _validate_path
    raise PathValidationError(pretty_message(
certvalidator.errors.PathValidationError: The path could not be validated because the end-entity certificate expired 2022-07-27 12:00:00Z
```

Author: fanquake

contrib/guix/manifest.scm

@@ -409,6 +409,11 @@ thus should be able to compile on most platforms where these exist.")
                   (string-append indent
                                  "@unittest.skip(\"Disabled by Guix\")\n"
                                  line)))
+               (substitute* "tests/test_validate.py"
+                 (("^(.*)def test_revocation_mode_soft" line indent)
+                  (string-append indent
+                                 "@unittest.skip(\"Disabled by Guix\")\n"
+                                 line)))
                #t))
            (replace 'check
              (lambda _