ci: refactor arm64 linux detection (#335)

Author: eitsupi

.github/workflows/release-lib.yml

@@ -75,7 +75,7 @@ jobs:
         uses: r-lib/actions/setup-r@v2
         with:
           # TODO: `true` after rspm supports arm linux <https://github.com/r-lib/actions/issues/960>
-          use-public-rspm: ${{ matrix.os != 'ubuntu-22.04-arm' }}
+          use-public-rspm: ${{ runner.os != 'Linux' || runner.arch != 'ARM64' }}
           Ncpus: 2
 
       - uses: r-lib/actions/setup-r-dependencies@v2
@@ -177,7 +177,7 @@ jobs:
         with:
           r-version: ${{ matrix.r }}
           # TODO: `true` after rspm supports arm linux <https://github.com/r-lib/actions/issues/960>
-          use-public-rspm: ${{ matrix.os != 'ubuntu-22.04-arm' }}
+          use-public-rspm: ${{ runner.os != 'Linux' || runner.arch != 'ARM64' }}
           Ncpus: "2"
 
       - uses: r-lib/actions/setup-r-dependencies@v2