GHA: Test also with R_FUTURE_BACKEND_VERSION=2

Author: HenrikBengtsson

.github/workflows/R-CMD-check.yaml

@@ -31,6 +31,7 @@ jobs:
           - {os: ubuntu-latest,  r: 'release'  , language: ko,    label: ko    }
           - {os: ubuntu-latest,  r: 'release'  , language: zh_CN, label: zh_CN }
           - {os: ubuntu-latest,  r: 'release'  , language: zh_TW, label: zh_TW }
+          - {os: ubuntu-latest,  r: 'release'  , backend_version: 2, label: 'multisession, backend_version=2' }
           - {os: ubuntu-latest,  r: 'release'  , globals_keepWhere: true, label: 'keepWhere' }
           - {os: ubuntu-latest,  r: 'release'  , globals_keepWhere: false, label: '!keepWhere' }
           - {os: ubuntu-latest,  r: 'release'  , plan: multicore, fork_multithreading_enable: false, label: 'multicore, no-multithreading-in-forks' }
@@ -55,6 +56,7 @@ jobs:
       ## Specific to futures
       R_FUTURE_RNG_ONMISUSE: error
       R_FUTURE_GLOBALS_KEEPWHERE: ${{ matrix.config.globals_keepWhere }}
+      R_FUTURE_BACKEND_VERSION: ${{ matrix.config.backend_version }}
 
     steps:
       - uses: actions/checkout@v4