Add linux sysroot package.

Project/Packages/manifest.json

     "com.unity.modules.physics": "1.0.0",
     "com.unity.modules.physics2d": "1.0.0",
     "com.unity.modules.uielements": "1.0.0",
-    "com.unity.modules.unityanalytics": "1.0.0"
+    "com.unity.modules.unityanalytics": "1.0.0",
+    "com.unity.sysroot.linux-x86_64": "0.1.15-preview"
   },
   "testables": [
     "com.unity.ml-agents",

Project/Packages/packages-lock.json

       "url": "https://packages.unity.com"
     },
     "com.unity.sysroot.linux-x86_64": {
-      "version": "0.1.14-preview",
-      "depth": 1,
+      "version": "0.1.15-preview",
+      "depth": 0,
-        "com.unity.sysroot": "0.1.18-preview"
+        "com.unity.sysroot": "0.1.19-preview"
       },
       "url": "https://packages.unity.com"
     },