Fix load path for precompiled code on Windows

[andyundso]

Closes #581.

The precompiled FreeTDS binary is located in the bin folder on Windows instead of lib. So when loading tiny_tds on Windows, an error was thrown, as the precompiled code was not found. There is a step in our CI that should have catched this error, as it does install tiny_tds from the built .gem-file and checks if it loads successfully. But this CI did not fail. Reason is that we also always cloned the entire source code for tiny_tds from Git, so likely irb just invoked the source code installed of the installed gem.

This PR:

• Removes the checkout step for two installation scenarios on Windows and the one on Linux
• Adds bin to the dll loads path when requiring tiny_tds

[aharpervc]

Re-running the single failed job, hopefully an ephemeral failure