Clean up + Merge composite actions (setup/build/test) into a single workflow

Making them reusable workflows are not possible because they would
not be callable as a single job step (which was what composite actions
are for). But the steps in these actions are so tiny and problem-specific
that making them standalone is hard to maintain anyway, so a single,
moderate-sized workflow is acceptable.

Author: leofang

.github/actions/build/action.yml

@@ -15,16 +15,11 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        # TODO: align host-platform names with conda convention
         host-platform:
           - linux-x64
           - linux-aarch64
           - win-x64
-        target-device:
-          - gpu
-        build-mode:
-          - release
-        upload-enabled:
-          - false
         python-version:
           - "3.13"
           - "3.12"
@@ -40,10 +35,6 @@ jobs:
       ./.github/workflows/gh-build-and-test.yml
     with:
       host-platform: ${{ matrix.host-platform }}
-      target-device: ${{ matrix.target-device }}
-      build-mode: ${{ matrix.build-mode }}
-      build-type: ci
-      upload-enabled: ${{ matrix.upload-enabled }}
       python-version: ${{ matrix.python-version }}
       cuda-version: ${{ matrix.cuda-version }}
     secrets: inherit