What does this PR do?

Switches the Windows release-artifact build over to the shared builder crate, so all platforms (Linux/glibc, Linux/musl, macOS, Windows) now go through the same path. To make that possible:

• builder/src/arch/windows.rs: adds Windows support to the builder — copies the PDB and the *.dll.lib import library, uses the _ffi-suffixed artifact names produced by the FFI crate, and no-ops the pkg-config step (Windows has no
  pkg-config).
• builder/src/arch/mod.rs + profiling.rs (refactor: move add_pkg_config to arch): hoists the per-platform add_pkg_config into each arch/*.rs, with the shared file-templating logic exposed as arch::generate_pkg_config. This is
  what lets Windows opt out cleanly instead of carrying a #[cfg] in profiling.rs.
• builder/src/builder.rs: skips creating bin/ and pkgconfig/ directories on Windows, where they're unused.
• windows/build-artifacts.ps1: deleted — its responsibilities now live in the builder.
• CI cleanup (separate commits, batched in because they unblock the Windows-on-builder rollout):
  • cargo nextest run invocations now pass --no-tests=pass so the workflow doesn't fail when a partition/affected-crate set yields zero tests.
  • .github/workflows/test.yml no longer hand-filters builder / test_spawn_from_lib out of the package list — --no-tests=pass makes that filtering unnecessary, and dropping it means the builder crate's own tests are actually
    exercised by CI when it changes.
  • Same fix applied to miri.yml and coverage.yml.

Motivation

We've been maintaining two parallel build pipelines: every other platform goes through the builder crate, while Windows had its own PowerShell script (windows/build-artifacts.ps1) that re-implemented overlapping logic (artifact naming,
output layout, header post-processing). Any change to how artifacts are produced had to be made (and kept in sync) in two places. Routing Windows through the builder consolidates this and lets future build changes ship in one place.

The CI changes are required collateral: once builder is the path used for the Windows release build, the workflow's hard-coded --exclude builder is wrong, and partition-style invocations need --no-tests=pass so the workflow tolerates
empty test sets.

Additional Notes

• Companion change in libddprof-build: this PR depends on a matching branch in the private libddprof-build repo. The last commit (dfeffd0b4 chore: reference libddprof branch that matches the changes in the builder) temporarily points
  .gitlab-ci.yml's DOWNSTREAM_BRANCH at that branch. This commit must be reverted to main before merge — please flag this in review if I forget.
• Windows artifact names move from datadog_profiling.{dll,lib,pdb} to datadog_profiling_ffi.{dll,lib,pdb} plus the new datadog_profiling_ffi.dll.lib import library. Downstream consumers that hard-coded the old names will need to follow.
• No changes to public APIs or to non-Windows artifact layout.