Expand Linux test matrix: (#4454)

This change makes progress on the plan in #4371. It does not replicate
the full [matrix] implemented in #3851, but it does replicate the 1.ii
section of the Linux matrix. It leverages "heavy" self-hosted runners,
and demonstrates a repeatable pattern for future matrices.

[matrix]: d794a0f3f1/.github/README.md (continuous-integration)

.github/actions/build/action.yml

@@ -0,0 +1,42 @@
+name: build
+inputs:
+  generator:
+    default: null
+  configuration:
+    required: true
+  cmake-args:
+    default: null
+# An implicit input is the environment variable `build_dir`.
+runs:
+  using: composite
+  steps:
+    - name: export custom recipes
+      shell: bash
+      run: conan export external/snappy snappy/1.1.9@
+    - name: install dependencies
+      shell: bash
+      run: |
+        mkdir ${build_dir}
+        cd ${build_dir}
+        conan install \
+          --output-folder . \
+          --build missing \
+          --settings build_type=${{ inputs.configuration }} \
+          ..
+    - name: configure
+      shell: bash
+      run: |
+        cd ${build_dir}
+        cmake \
+          ${{ inputs.generator && format('-G {0}', inputs.generator) || '' }} \
+          -DCMAKE_TOOLCHAIN_FILE:FILEPATH=build/generators/conan_toolchain.cmake \
+          -DCMAKE_BUILD_TYPE=${{ inputs.configuration }} \
+          ${{ inputs.cmake-args }} \
+          ..
+    - name: build
+      shell: bash
+      run: |
+        cmake \
+          --build ${build_dir} \
+          --config ${{ inputs.configuration }} \
+          --parallel ${NUM_PROCESSORS:-$(nproc)}