feat: graph-to-tree: set .targetOS also for linux pkg-manager

Imported scratch images have linux as a package manager
DepTree should include targetOS.

src/legacy/index.ts

@@ -251,7 +251,7 @@ function constructPackageFormatVersion(pkgType: string): string {
 }
 
 function constructTargetOS(depGraph: types.DepGraph): { name: string; version: string; } | void {
-  if (['apk', 'apt', 'deb', 'rpm'].indexOf(depGraph.pkgManager.name) === -1) {
+  if (['apk', 'apt', 'deb', 'rpm', 'linux'].indexOf(depGraph.pkgManager.name) === -1) {
     // .targetOS is undefined unless its a linux pkgManager
     return;
   }

test/fixtures/os-linux-scratch-dep-graph.json

@@ -0,0 +1,30 @@
+{
+    "schemaVersion": "1.2.0",
+    "pkgManager": {
+      "name": "linux",
+      "repositories": [
+        {
+          "alias": "unknown:0.0"
+        }
+      ]
+    },
+    "pkgs": [
+      {
+        "id": "docker-image|scratch@0",
+        "info": {
+          "name": "docker-image|scratch",
+          "version": "0"
+        }
+      }
+    ],
+    "graph": {
+      "rootNodeId": "root-node",
+      "nodes": [
+        {
+          "nodeId": "root-node",
+          "pkgId": "docker-image|scratch@0",
+          "deps": []
+        }
+      ]
+    }
+  }

test/fixtures/os-linux-scratch-dep-tree.json

@@ -0,0 +1,10 @@
+{
+    "name": "docker-image|scratch",
+    "version": "0",
+    "type": "linux",
+    "packageFormatVersion": "linux:0.0.1",
+    "targetOS": {
+      "name": "unknown",
+      "version": "0.0"
+    }
+  }

test/legacy/to-dep-tree.test.ts

@@ -45,6 +45,12 @@ describe('dep-trees survive serialisation through dep-graphs', () => {
       pkgManagerName: 'rpm',
       pkgType: 'rpm',
     },
+    {
+      description: 'os dep-tree (linux - scratch image)',
+      path: 'os-linux-scratch-dep-tree.json',
+      pkgManagerName: 'linux',
+      pkgType: 'linux',
+    },
     {
       description: 'maven dep-tree',
       path: 'maven-dep-tree.json',