refactor: lake: libPrefixOnWindows on libName

[tydeu]

This PR alters libPrefixOnWindows behavior to add the lib prefix to the library's libName rather than just the file path. This means that Lake's -l will now have the prefix on Windows. While this should not matter to a MSYS2 build (which accepts both lib-prefixed and unprefixed variants), it should ensure compatibility with MSVC (if that is ever an issue).

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2025-09-26 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2025-09-27 00:03:03)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-09-26 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2025-09-27 00:03:05)