Add github workflow to build Windows (64bit) binary

[milkpirate]

Description of your changes

To make git-crypt easier to use on Windows I added a workflow which builds the Win64 binary via MSYS in the CI. To do so I:

• added a workflow
• modified the Makefile a little (pertty much what @LykkeCity did in https://github.com/LykkeCity/git-crypt)

How has this code been tested?

• ran the downloaded artifact on Windows 10 successfully

Tasks to do

• [x] Read contributing guidelines contributiing guidlines.

[milkpirate]

Close PR in favour of https://github.com/AGWA/git-crypt/pull/227