Support setting shell via defaults.run (#343)

Note that we don't support the more general use of defaults, just its
run key for setting shell and working directory.

The documentation for defaults.run is here:
  https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#defaultsrun

Fixes #214.

Co-authored-by: Casey Lee <cplee@nektos.com>

act/model/workflow.go

@@ -13,10 +13,11 @@ import (
 
 // Workflow is the structure of the files in .github/workflows
 type Workflow struct {
-	Name  string            `yaml:"name"`
-	RawOn yaml.Node         `yaml:"on"`
-	Env   map[string]string `yaml:"env"`
-	Jobs  map[string]*Job   `yaml:"jobs"`
+	Name     string            `yaml:"name"`
+	RawOn    yaml.Node         `yaml:"on"`
+	Env      map[string]string `yaml:"env"`
+	Jobs     map[string]*Job   `yaml:"jobs"`
+	Defaults Defaults          `yaml:"defaults"`
 }
 
 // On events for the workflow
@@ -64,6 +65,7 @@ type Job struct {
 	Services       map[string]*ContainerSpec `yaml:"services"`
 	Strategy       *Strategy                 `yaml:"strategy"`
 	RawContainer   yaml.Node                 `yaml:"container"`
+	Defaults       Defaults                  `yaml:"defaults"`
 }
 
 // Strategy for the job
@@ -73,6 +75,17 @@ type Strategy struct {
 	Matrix      map[string][]interface{} `yaml:"matrix"`
 }
 
+// Default settings that will apply to all steps in the job or workflow
+type Defaults struct {
+	Run RunDefaults `yaml:"run"`
+}
+
+// Defaults for all run steps in the job or workflow
+type RunDefaults struct {
+	Shell            string `yaml:"shell"`
+	WorkingDirectory string `yaml:"working-directory"`
+}
+
 // Container details for the job
 func (j *Job) Container() *ContainerSpec {
 	var val *ContainerSpec