Migrate fixtures from buildpack.yml to env var configuration

[fg-j]

What happened?

• What were you attempting to do?
  I was looking around at the fixtures to explore ways of setting up dotnet apps.

• What did you expect to happen?
  Given that buildpack.yml has been deprecated, I expected most of the fixtures to use a project.toml instead of a buildpack.yml.

• What was the actual behavior? Please provide log output, if possible.
  But many of the fixtures still rely on a buildpack.yml because the work to move dotnet-execute away from buildpack.yml isn't finished yet.

Once paketo-buildpacks/dotnet-execute#159 is merged and complete, the fixtures here should be updated to remove the buildpack.yml

Build Configuration

• What platform (pack, kpack, tekton buildpacks plugin, etc.) are you
  using? Please include a version.

• What buildpacks are you using? Please include versions.

• What builder are you using? If custom, can you provide the output from pack inspect-builder <builder>?

• Can you provide a sample app or relevant configuration (buildpack.yml,
  nginx.conf, etc.)?

Checklist

• I have included log output.
• The log output includes an error message.
• I have included steps for reproduction.

[fg-j]

Blocked by paketo-buildpacks/dotnet-execute#159

[fg-j]

Resolved by #176