feat: tart macOS vm's as job container (#33)

adds the tart:// protocol to platform mapping

e.g. `-P macos-14=tart://ghcr.io/cirruslabs/macos-sonoma-base:latest` if you have a mac.

`add-path` is probably broken

pkg/tart/env_darwin.go

@@ -0,0 +1,18 @@
+package tart
+
+type Env struct {
+	JobID           string
+	JobImage        string
+	FailureExitCode int
+	Registry        *Registry
+}
+
+type Registry struct {
+	Address  string
+	User     string
+	Password string
+}
+
+func (e Env) VirtualMachineID() string {
+	return e.JobID
+}