OSS CI: emit PIC + THRIFT_SERIALIZATION_ONLY cmake defines

Summary:
The hand-maintained workflow yml in the rebalancer OSS GitHub repo passes `CMAKE_POSITION_INDEPENDENT_CODE: ON` to every dep build (boost, clang19, fmt, folly, fbthrift, …) and `THRIFT_SERIALIZATION_ONLY: ON` to fbthrift. The fbsource-side regenerated yml drops these on every regen, requiring repeated hand-fixes to keep CI green.

This change makes the regen produce the same defines:

- `update-all-github-actions.sh`: pass `--extra-cmake-defines '{"CMAKE_POSITION_INDEPENDENT_CODE":"ON"}'` and `--package-extra-cmake-defines fbthrift='{"THRIFT_SERIALIZATION_ONLY":"ON"}'` to the rebalancer linux + darwin generate-github-actions invocations.
- `getdeps.py` (`generate-github-actions`): seed the per-step `extra_cmake_defines` dict from `args.extra_cmake_defines` so the existing flag actually propagates to dep build steps. Previously the flag was registered but ignored at the generator level — the dict was always re-initialized to empty before being merged with sccache + per-package overrides. This matches the flag's documented semantics ('current project and all its deps').

Only the rebalancer GEN lines pass the new flag, so other projects' workflows are unaffected.

The README.facebook.md tweak drops the literal '`generated`' rendering since these files do not actually carry the generated marker.

Differential Revision: D103882417

fbshipit-source-id: 5cf428eb688e7c51ff600d923a73a9ddc824c27c

Author: r-barnes

build/fbcode_builder/getdeps.py

@@ -1244,7 +1244,9 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
 
             out.write("    - uses: actions/checkout@v6\n")
 
-            extra_cmake_defines = {}
+            extra_cmake_defines = (
+                json.loads(args.extra_cmake_defines) if args.extra_cmake_defines else {}
+            )
             if use_sccache:
                 out.write("    - name: Set up sccache\n")
                 out.write("      uses: mozilla-actions/sccache-action@v0.0.9\n")