ci: exclude YAML conformance fixtures

Use yamllint's native ignore rules for vendored spec and invalid parser fixtures without relying on unsupported reusable-workflow environment settings.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug

.github/PSModule.yml

@@ -23,7 +23,6 @@ Build:
 
 Linter:
   env:
-    FILTER_REGEX_EXCLUDE: '.*tests/fixtures/.*'
     VALIDATE_BIOME_FORMAT: false
     VALIDATE_BIOME_LINT: false
     VALIDATE_GITHUB_ACTIONS_ZIZMOR: false

.github/linters/.yaml-lint.yml

@@ -0,0 +1,6 @@
+---
+extends: relaxed
+
+# Official conformance fixtures intentionally preserve invalid and unusual YAML.
+ignore: |
+  tests/fixtures/