CI: drop defaults channel from conda installs to fix mixed-channel solve

The macOS conda env solve has been intermittently failing with
LibMambaUnsatisfiableError because conda-env-ci.txt's `cmake=3.31.2`
needs newer transitive deps from conda-forge (libzlib>=1.3.1,
rhash>=1.4.5) than the miniconda base env's anaconda-defaults pulls in
(zlib=1.2.13, rhash=1.4.3). The Voxtral metal e2e ffmpeg install hit
the same class of conflict.

A previous attempt added `--strict-channel-priority`, but libmamba on
macOS emits `Problem type not implemented SOLVER_RULE_STRICT_REPO_PRIORITY`
and falls into a deterministic failure on the rhash conflict — that
flag is effectively unimplemented for this code path. `conda config
--set channel_priority strict` would land in the same libmamba code
path and additionally leak `~/.condarc` state across the persistent
self-hosted macOS runners.

`--override-channels` is implemented at the conda CLI layer (filters
the channel list before the solver runs) rather than as a solver rule,
so it works with libmamba. Dropping defaults is safe for these call
sites: every package they install (cmake, ninja, libuv, llvm-openmp,
pkg-config, ffmpeg, libstdcxx-ng, openblas) is first-class on conda-
forge, and downstream pip-managed deps are unaffected. Patch all five
mixed-channel `conda install` sites in the repo for consistency.

Authored with Claude Code.

Author: rascani

.ci/docker/common/install_conda.sh

@@ -39,7 +39,7 @@ install_python() {
 
   # From https://github.com/pytorch/pytorch/blob/main/.ci/docker/common/install_conda.sh
   if [[ $(uname -m) == "aarch64" ]]; then
-    conda_install "openblas==0.3.29=*openmp*" -c conda-forge
+    conda_install "openblas==0.3.29=*openmp*" -c conda-forge --override-channels
   else
     conda_install mkl=2022.1.0 mkl-include=2022.1.0
   fi

.ci/scripts/setup-conda.sh

@@ -9,7 +9,14 @@ set -ex
 
 install_conda() {
   pushd .ci/docker || return
-  ${CONDA_INSTALL} -c conda-forge -y --file conda-env-ci.txt
+  # --override-channels excludes the anaconda defaults channel so the
+  # solve doesn't have to reconcile defaults' older transitive deps
+  # (e.g. zlib=1.2.13, rhash=1.4.3) with conda-forge's newer ones
+  # required by cmake=3.31.2 (libzlib>=1.3.1, rhash>=1.4.5). Mixing
+  # the two channels intermittently failed with LibMambaUnsatisfiable
+  # Error; libmamba on macOS does not implement strict channel
+  # priority, so the only deterministic fix is to drop defaults.
+  ${CONDA_INSTALL} -c conda-forge --override-channels -y --file conda-env-ci.txt
   popd || return
 }
 

.ci/scripts/test_model_e2e.sh

@@ -258,7 +258,7 @@ fi
 if [ "$AUDIO_URL" != "" ]; then
   curl -L $AUDIO_URL -o ${MODEL_DIR}/$AUDIO_FILE
 elif [[ "$MODEL_NAME" == *whisper* ]] || [ "$MODEL_NAME" = "voxtral_realtime" ]; then
-  conda install -y -c conda-forge "ffmpeg<8"
+  conda install -y -c conda-forge --override-channels "ffmpeg<8"
   pip install datasets soundfile
   # We pushd'd into EXECUTORCH_ROOT above, so torch_pin is importable here.
   TORCHCODEC_PKG=$(python -c "from torch_pin import torchcodec_spec; print(torchcodec_spec())")

.github/workflows/cuda.yml

@@ -125,7 +125,7 @@ jobs:
 
         # The Triton-compiled .so files in the CUDA backend require GLIBCXX_3.4.29
         # which the default system libstdc++ doesn't have. Install a newer one.
-        conda install -y -c conda-forge 'libstdcxx-ng>=12'
+        conda install -y -c conda-forge --override-channels 'libstdcxx-ng>=12'
         export LD_LIBRARY_PATH=/opt/conda/lib:$LD_LIBRARY_PATH
 
         # Build ExecuTorch with CUDA support
@@ -433,7 +433,7 @@ jobs:
         # The embedded .so files in the CUDA blob require GLIBCXX_3.4.29
         # which the default conda libstdc++ doesn't have. Install a newer
         # libstdc++ from conda-forge and use it via LD_PRELOAD.
-        conda install -y -c conda-forge 'libstdcxx-ng>=12'
+        conda install -y -c conda-forge --override-channels 'libstdcxx-ng>=12'
         export LD_LIBRARY_PATH=/opt/conda/lib:$LD_LIBRARY_PATH
         # Verify the new libstdc++ has GLIBCXX_3.4.29
         strings /opt/conda/lib/libstdc++.so.6 | grep GLIBCXX_3.4.29 || {