Pin abseil in the build environment too

<details><summary>Claude's draft</summary>

libprotobuf 7.35.1 build _4 (from the abseil 20260817 migration) pulls
libabseil 20260817 into the build environment, while the host stays on
the pinned 20260526. On Windows, CMake also searches the prefixes of
PATH entries, and patch 0011 lets find_package(absl) accept any version,
so it can pick up the build environment's abseil:

- win CUDA: compiled against 20260817 headers, linked against host
  20260526 → LNK2001 absl::lts_20260817::ThrowStdLengthError.
- win CPU: the tests load the build env's abseil_dll at run time →
  0xc0000139 (entry point not found) in 15 of 18 ctest tests.

A bare libabseil in build is pinned by the variant to the host version,
so the solver picks a protoc built against it.

Resume this Claude session:
```
cd /home/mark/git/feedstock
claude --resume fdaba65c-5a43-4f26-a32f-d49b747fdbd7
```
</details>

Claude-Session: https://claude.ai/code/session_01R9WZp6UVaiXYyC2hWTNWZY

Author: hmaarrfk

recipe/recipe.yaml

@@ -95,6 +95,7 @@ outputs:
         - nushell
         # protoc must run on the build platform
         - libprotobuf
+        - libabseil
         - if: build_platform != target_platform
           then:
             - python ${{ staging_python }}.*
@@ -254,6 +255,7 @@ outputs:
         - ninja
         - nushell
         - libprotobuf
+        - libabseil
         - if: build_platform != target_platform
           then:
             - python