Enable CI tests against Windows

[omus]

When working on #643 I ended up writing the _whoami function which I wanted to verify would work against Windows. When running the CI test on Windows I noticed a few issues with our test setup which stopped it working on Windows including:

• Our MinIO test setup
• An incompatibility with our Credential Process testset:

   ~/.aws/config - Credential Process: Error During Test at D:\a\AWS.jl\AWS.jl\test\AWSCredentials.jl:840
    Got exception outside of a @test
    IOError: could not spawn `'C:UsersRUNNER~1AppDataLocalTempjl_hHzm8Pcred_process'`: no such file or directory (ENOENT)

We originally disabled the Windows CI job back in #255 when we switched from Travis CI to using GHA. Although undocumented I believe this was originally this was done to reduce waiting on the slower Windows CI jobs. The Windows CI jobs are still unfortunately slow so maybe a nice compromise here would be to only run the Windows CI jobs against the default branch. At least with that setup we'll be able to notice Windows incompatibilities that have crept into our code and tests.