Fix bugs on Windows and 64-bit architectures
Fix bugs on Windows and 64-bit architectures

- Correctly fetch Bazel releases ending with ".exe" rather than without
  when downloading on Windows.
- Recognise "amd64" and "AMD64" machine architectures as aliases for
  "x86_64" when downloading Bazel.

bazelbuild/bazelisk,bazelbuild/bazelisk,bazelbuild/bazelisk,bazelbuild/bazelisk