chore: Limits CI build and test parallelism to reduce resource contention (#5799)

GitHub runners have a limit on how many concurrent jobs they can actually process (even though they will try to run them all at the same time), and similarly the Conan remote cannot handle hundreds of concurrent requests. Previously, the Conan dependency uploading was already limited to max 10 jobs running in parallel, and this change makes the same change to the build+test workflow.

.github/workflows/build-test.yml

@@ -61,6 +61,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix: ${{ fromJson(needs.generate-matrix.outputs.matrix) }}
+      max-parallel: 10
     runs-on: ${{ matrix.architecture.runner }}
     container: ${{ inputs.os == 'linux' && format('ghcr.io/xrplf/ci/{0}-{1}:{2}-{3}', matrix.os.distro_name, matrix.os.distro_version, matrix.os.compiler_name, matrix.os.compiler_version) || null }}
     steps: