conformance: update hashicorp fork to skip failing tests

[mikemorris]

Changes proposed in this PR:

Update the conformance testing workflow to checkout an updated branch on the hashicorp/gateway-api fork with a few failing tests skipped.

The longer-term plan will be to integrate the conformance suite directly as in #205 rather than relying on a fork, but we're holding off on that until gateway-api tags a v0.5.0 release.

How I've tested this PR:

Observed the conformance tests are now passing.

How I expect reviewers to test this PR:

Verify that this change is an adequate short-term resolution.

Checklist:

• Tests added
• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[mikemorris]

Reopened this due to two tests timing out over in #217 which appear to still need the 60s window.

     --- FAIL: TestConformance/HTTPRouteMatchingAcrossRoutes (1.04s)
        --- FAIL: TestConformance/HTTPRouteMatchingAcrossRoutes/7_request_to_example.com/_with_headers_should_go_to_infra-backend-v2 (30.00s)
        --- FAIL: TestConformance/HTTPRouteMatchingAcrossRoutes/0_request_to_example.com/_should_go_to_infra-backend-v1 (30.00s)