fix: allow using the env context in actions defaults (#204)

Reviewed-on: https://code.forgejo.org/forgejo/act/pulls/204
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/action_schema.json

@@ -166,6 +166,7 @@
       "context": [
         "forge",
         "github",
+        "env",
         "strategy",
         "matrix",
         "job",

act/schema/schema_test.go

@@ -120,3 +120,32 @@ jobs:
 	}
 	require.NoError(t, n.UnmarshalYAML(&node))
 }
+
+func TestActionSchema(t *testing.T) {
+	var node yaml.Node
+	err := yaml.Unmarshal([]byte(`
+name: 'action name'
+author: 'action authors'
+description: |
+  action description
+inputs:
+  url:
+    description: 'url description'
+    default: '${{ env.GITHUB_SERVER_URL }}'
+  repo:
+    description: 'repo description'
+    default: '${{ github.repository }}'
+runs:
+  using: "composite"
+  steps:
+    - run: echo "${{ github.action_path }}"
+`), &node)
+	if !assert.NoError(t, err) {
+		return
+	}
+	err = (&Node{
+		Definition: "action-root",
+		Schema:     GetActionSchema(),
+	}).UnmarshalYAML(&node)
+	assert.NoError(t, err)
+}