Fix osx-arm64 conda build: drop redundant openblas pin, allow single-platform release dispatch (#5652)

Summary:
The `OSX arm64 packages` job failed in the v1.15.1 release run ([35131196327](https://github.com/facebookresearch/faiss/actions/runs/35131196327)), so `faiss-cpu` 1.15.1 is on the pytorch channel for linux-64, linux-aarch64 and win-64 but **not** osx-arm64. PyPI is unaffected.

### Root cause

Upstream conda-forge inconsistency, not a faiss regression. On 2026-09-10 conda-forge published two new `libopenblas 0.3.34` builds for osx-arm64 without matching `openblas 0.3.34` metapackages:

| `openblas 0.3.34` accepts | `libopenblas 0.3.34` available |
|---|---|
| `openmp_he657e61_0` | `openmp_he657e61_0` (07-17) |
| `pthreads_hddb8425_0` | `pthreads_hddb8425_0` (07-17) |
| `pthreads_h60d1960_1` | `pthreads_h60d1960_1` (08-30) |
| `openmp_h4f80526_1` | `openmp_h4f80526_1` (08-30) |
| — | `pthreads_hb42d564_1` (09-10) ← no `openblas` |
| — | `openmp_h5e6e99c_1` (09-10) ← no `openblas` |

`libfaiss` built fine against `openmp_h4f80526_1`. The failure came when solving the `faiss-cpu` env: the unpinned `libopenblas =0.3.34` resolves to the newest build `openmp_h5e6e99c_1`, which no `openblas 0.3.34` accepts:

```
package libfaiss-1.15.1-py3.12_hcb8d3e5_0_cpu requires openblas 0.3.34.*,
but none of the providers can be installed
```

The recipe is vulnerable because the `libfaiss` output pins **both** `openblas` and `libopenblas` on osx-arm64 — `# [not x86_64]` was written for linux-aarch64 but also matches macOS ARM. The `faiss-cpu` output in the same file already pins only `libopenblas` on osx.

### Changes

1. **`conda/faiss/meta.yaml`** (lines 72, 86) — narrow the selector to `# [linux and not x86_64]`, so osx-arm64 constrains only `libopenblas` (what `libfaiss` actually links: `libopenblas.0.dylib`). linux-aarch64 and x86_64 behavior unchanged.

2. **`.github/workflows/build-release.yml`** — add `workflow_dispatch` with a `platforms` choice input. The workflow was `workflow_call`-only, so a single failed conda leg could not be rebuilt without pushing a tag. Each job gains an `if:` that is a no-op on tag pushes (`github.event_name` is `push`), preserving current release behavior.

### Recovery plan for 1.15.1

After this lands, dispatch `build-release.yml` from `main` with `platforms: osx-arm64`. The conda version comes from `git describe`, so this produces **1.15.1 build 1** — same version, next build number, no retagging and no PyPI involvement.

### Note

If conda-forge publishes the missing `openblas` builds, the old recipe would start working again on its own. This change removes the coupling so the build no longer depends on that.


Test Plan:
- [ ] CI passes
- [ ] Dispatch `build-release.yml` with `platforms: osx-arm64` and confirm the conda solve succeeds
- [ ] Confirm `faiss-cpu` 1.15.1 appears for osx-arm64 on the pytorch channel

Differential Revision: D120400090

Pulled By: alibeklfc

conda/faiss/meta.yaml

@@ -69,7 +69,7 @@ outputs:
         - mkl >=2024.2.2,<2026  # [x86_64 and win]
         - python_abi =3.12
         {% endif %}
-        - openblas =0.3.34  # [not x86_64]
+        - openblas =0.3.34  # [linux and not x86_64]
         - libopenblas =0.3.34  # [osx]
       run:
         - python {{ python }}
@@ -83,7 +83,7 @@ outputs:
         - mkl >=2024.2.2,<2026  # [x86_64 and win]
         - python_abi =3.12
         {% endif %}
-        - openblas =0.3.34  # [not x86_64]
+        - openblas =0.3.34  # [linux and not x86_64]
         - libopenblas =0.3.34  # [osx]
     test:
       requires: