Version Used: Roslyn 2.8.0 (e595ee2)

The regression was introduced with e2fc8d5 / PR #25426.

Steps to Reproduce:

1. Try using full signing on Mono on Unix: csc /keyfile:mykey.snk Program.cs
2. It prints an mscoree.dll error

Expected Behavior:

No error.

Actual Behavior:

error CS7027: Error signing output with public key from file 'mykey.snk' -- mscoree.dll

The new managed signing needs to be enabled when running on Mono too, not just on CoreCLR.

/cc @agocke