fix: schema validation must accept aliases and merges (#1012)

- aliases are an indirection to follow
  https://yaml.org/spec/1.2.1/#id2785586
- merges cannot be conveniently validated and are skipped
  https://yaml.org/type/merge.html

Resolves forgejo/runner#1011

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- bug fixes
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/1012): <!--number 1012 --><!--line 0 --><!--description Zml4OiBzY2hlbWEgdmFsaWRhdGlvbiBtdXN0IGFjY2VwdCBhbGlhc2VzIGFuZCBtZXJnZXM=-->fix: schema validation must accept aliases and merges<!--description-->
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Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/1012
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

act/schema/schema_test.go

@@ -243,3 +243,39 @@ runs:
 		})
 	}
 }
+
+// https://yaml.org/spec/1.2.1/#id2785586
+// An anchor is denoted by the “&” indicator. It marks a node for future reference.
+// https://yaml.org/type/merge.html
+// Specify one or more mappings to be merged with the current one.
+func TestSchema_AnchorAndReference(t *testing.T) {
+	var node yaml.Node
+	err := yaml.Unmarshal([]byte(`
+on: [push]
+jobs:
+  test1:
+    runs-on: docker
+    steps:
+      - &step
+        run: echo All good!
+      - *step
+  test2:
+    runs-on: docker
+    steps:
+      - << : *step
+        name: other name
+  test3:
+    runs-on: docker
+    steps:
+      - !!merge << : *step
+        name: other name
+`), &node)
+	if !assert.NoError(t, err) {
+		return
+	}
+	err = (&Node{
+		Definition: "workflow-root",
+		Schema:     GetWorkflowSchema(),
+	}).UnmarshalYAML(&node)
+	assert.NoError(t, err)
+}