Add PYOQS_VERSION env var to override auto-installed liboqs version

[dstebila]

By default liboqs-python auto-installs the liboqs release matching its own version. Setting PYOQS_VERSION overrides that target:

PYOQS_VERSION=0.15.0 # install a specific liboqs release
PYOQS_VERSION=latest # install liboqs from main (HEAD)

The override applies only at the install site (OQS_VERSION). The oqs_python_version() helper is unchanged, so the liboqs vs. liboqs-python version-mismatch warning continues to report the actually-installed package version rather than the override.

Closes #132, #133.

[dstebila]

@Guiliano99 I'm preparing the repository for a new release, to match the forthcoming liboqs 0.16.0. I extracted the PYOQS_VERSION code from your PR #132 into a separate PR.

[Guiliano99]

@dstebila Hi, thanks — and thanks for fixing #136. I do not have access to a Windows machine, so I did not catch that issue.

Regarding the STFL caching: I added keyutils-py to make key interactions easier, especially for PKIX-related environments.

Would it make sense to open a separate PR where the XMSS/XMSS^MT keys are loaded through that package instead of being cached directly? The package still relies on the liboqs code for these keys, so it would mainly simplify the setup in the pipeline.

[dstebila]

Regarding the STFL caching: I added keyutils-py to make key interactions easier, especially for PKIX-related environments.

Would it make sense to open a separate PR where the XMSS/XMSS^MT keys are loaded through that package instead of being cached directly? The package still relies on the liboqs code for these keys, so it would mainly simplify the setup in the pipeline.

I wouldn't want to add a new dependency to liboqs-python in general. Would this be only for testing? And perhaps we should move the discussion over to #130.