Remove DOTNET_ROLL_FORWARD in ci.tests.yml

https://github.com/Azure/azure-sdk-for-cpp/pull/5681/files#diff-9a817977293f806ff99e282fb84005c9d7d96a314a112fd977f42685fec32edaR187-R190

Even after merging `main` into this PR and removing the above linked task CI fails. Investigate, fix.

For now there's a workaround in place, remove the workaround when the fix is in. 