Pin Intel OneAPI to 2025.3 in CI workflows for coverage

[vlad-perevezentsev]

This PR proposes using OneAPI 2025.3 for build-sphinx and generate_coverage workflows to match the compiler version used to build dpctl packages since dpctl is waiting for the latest compiler to become available on conda-forge dpctl#2300

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2887/index.html

[coveralls]

Coverage is 78.444% — pin_coverage_docs_build into master. No base build found for master.

[antonwolfy]

It seems we need to pin only for coverage workflow due to known 2026.0 compiler issue with Segfault during coverage build

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev0=py313h509198e_15 ran successfully.
Passed: 1358
Failed: 2
Skipped: 16

[ndgrigorian]

coverage fixed, LGTM