Fix cross compilation and test in CI

[SWilson4]

The recent addition of the oqs-internal library broke cross compilation for Windows due to differing object visibility defaults for Linux and Windows.

This is pending our Focal container being updated to install gcc-mingw-w64.

I also took the opportunity to update our checkout workflows to v4, which will make the GitHub Actions deprecation warnings go away.

Fixes #1693.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[baentsch]

Thanks for the updates and new cross-compile CI job! LGTM -- if CI now triggered passes.

[SWilson4]

The container has been updated and the tests all pass, including the new ones for cross-compilation.