[DotNet Reference] document env var `BP_DOTNET_ROLL_FORWARD`

[joshuatcasey]

Problem

https://paketo.io/docs/reference/dotnet-core-reference/ does not have documentation for env var BP_DOTNET_ROLL_FORWARD, which was added in paketo-buildpacks/dotnet-core#385

Affected page(s)

• https://paketo.io/docs/reference/dotnet-core-reference/

Checklist

• I have included links to page(s) that need to be changed