CI: test with supported C++ standards (#1226)

* CI: test with supported C++ standards

let's test with the supported standards for better coverage.
C++23 standard was just out, but the support on the toolchain is
not quite ready yet. so let's leave it for a future change.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>

* CI: drop redundant Build step

"Build Tests" is strictly a superset of "Build". in addition to
the library, the former builds the tests also. both these
steps share the same set of command line arguments. by removing
"Build" step, we don't lose anything regarding the test coverage
and information for further investigation if the build fails.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>

---------
Signed-off-by: Kefu Chai <tchaikov@gmail.com>

.github/workflows/build.yml

@@ -12,6 +12,7 @@ jobs:
         fail-fast: false
         matrix:
           os: [ubuntu-latest, windows-latest, macos-latest]
+          cxx_standard: [11, 17, 20]
           build: [static, shared]
           generator: ["Default Generator", "MinGW Makefiles"]
           exclude:
@@ -23,7 +24,6 @@ jobs:
             generator: "MinGW Makefiles"
       env:
         YAML_BUILD_SHARED_LIBS: ${{ matrix.build == 'shared' && 'ON' || 'OFF' }}
-        YAML_CPP_BUILD_TESTS: 'ON'
         CMAKE_GENERATOR: >-
           ${{format(matrix.generator != 'Default Generator' && '-G "{0}"' || '', matrix.generator)}}
       runs-on: ${{ matrix.os }}
@@ -33,16 +33,10 @@ jobs:
         - name: Get number of CPU cores
           uses: SimenB/github-actions-cpu-cores@v1
 
-        - name: Build
-          shell: bash
-          run: |
-            cmake ${{ env.CMAKE_GENERATOR }} -S "${{ github.workspace }}" -B build -DYAML_BUILD_SHARED_LIBS=${{ env.YAML_BUILD_SHARED_LIBS }}
-            cd build && cmake --build . --parallel ${{ steps.cpu-cores.outputs.count }}
-            
         - name: Build Tests
           shell: bash
           run: |
-            cmake ${{ env.CMAKE_GENERATOR }} -S "${{ github.workspace }}" -B build -DYAML_BUILD_SHARED_LIBS=${{ env.YAML_BUILD_SHARED_LIBS }} -DYAML_CPP_BUILD_TESTS=${{ env.YAML_CPP_BUILD_TESTS }}
+            cmake ${{ env.CMAKE_GENERATOR }} -S "${{ github.workspace }}" -B build -DCMAKE_CXX_STANDARD=${{ matrix.cxx_standard }} -DYAML_BUILD_SHARED_LIBS=${{ env.YAML_BUILD_SHARED_LIBS }} -DYAML_CPP_BUILD_TESTS=ON
             cd build && cmake --build . --parallel ${{ steps.cpu-cores.outputs.count }}
 
         - name: Run Tests