Revert "CI: parallel build follow up" (#51984)

Revert "CI: parallel build follow up (#51911)"

This reverts commit 2c996a6.

.github/actions/build_pandas/action.yml

@@ -16,5 +16,7 @@ runs:
         python -m pip install -e . --no-build-isolation --no-use-pep517 --no-index
       shell: bash -el {0}
       env:
-        # https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources
-        N_JOBS: ${{ runner.os == 'macOS' && 3 || 2 }}
+        # Cannot use parallel compilation on Windows, see https://github.com/pandas-dev/pandas/issues/30873
+        # GH 47305: Parallel build causes flaky ImportError: /home/runner/work/pandas/pandas/pandas/_libs/tslibs/timestamps.cpython-38-x86_64-linux-gnu.so: undefined symbol: pandas_datetime_to_datetimestruct
+        N_JOBS: 1
+        #N_JOBS: ${{ runner.os == 'Windows' && 1 || 2 }}

.github/actions/setup-conda/action.yml

@@ -30,7 +30,7 @@ runs:
         environment-name: ${{ inputs.environment-name }}
         extra-specs: ${{ inputs.extra-specs }}
         channels: conda-forge
-        channel-priority: 'strict'
+        channel-priority: ${{ runner.os == 'macOS' && 'flexible' || 'strict' }}
         condarc-file: ci/condarc.yml
         cache-env: true
         cache-downloads: true