Fix testhost depproj chmod when win host restored (dotnet/corefx#34335)

Fix testhost depproj chmod when win host restored (dotnet/corefx#34335)

When building on Linux, this Exec assumes a Linux configuration is
restored so the testhost package it picks up will have dotnet. This
fails with '-allconfigurations' when a Windows configuration is used and
the restore actually got dotnet.exe. This change detects whatever dotnet
file was actually restored and runs chmod on that.

Commit migrated from https://github.com/dotnet/corefx/commit/70d08ae0b1f2487f0684a66be364a988f181567d