Bump Imageflow.NativeRuntime.ubuntu-x86_64 in /examples/StartHere

Bumps [Imageflow.NativeRuntime.ubuntu-x86_64](https://github.com/imazen/imageflow) from 2.0.0-preview1 to 2.0.0-preview8.
- [Release notes](https://github.com/imazen/imageflow/releases)
- [Commits](imazen/imageflow@v2.0.0-preview1...v2.0.0-preview8)

---
updated-dependencies:
- dependency-name: Imageflow.NativeRuntime.ubuntu-x86_64
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

examples/StartHere/packages.config

@@ -3,7 +3,7 @@
   <package id="Antlr" version="3.5.0.2" targetFramework="net48" />
   <package id="Imageflow.AllPlatforms" version="0.9.0" targetFramework="net48" />
   <package id="Imageflow.NativeRuntime.osx-x86_64" version="2.0.0-preview1" targetFramework="net48" />
-  <package id="Imageflow.NativeRuntime.ubuntu-x86_64" version="2.0.0-preview1" targetFramework="net48" />
+  <package id="Imageflow.NativeRuntime.ubuntu-x86_64" version="2.0.0-preview8" targetFramework="net48" />
   <package id="Imageflow.NativeRuntime.win-x86" version="2.0.0-preview1" targetFramework="net48" />
   <package id="Imageflow.NativeRuntime.win-x86_64" version="2.0.0-preview8" targetFramework="net48" />
   <package id="Imageflow.Net" version="0.9.0" targetFramework="net48" />