Fine-tune CI for better debugability

.github/workflows/main.yml

@@ -13,6 +13,7 @@ jobs:
     # Define the strategy for the build job. We generally want to cover each
     # platform that we support here
     strategy:
+      fail-fast: false
       matrix:
         platform: [generic, hifive_unmatched, cva6, mpfs]
         bits: [32, 64]
@@ -191,7 +192,7 @@ jobs:
       with:
         name: keystone-${{ matrix.platform }}${{ matrix.bits }}-builddir
         path: build.tar.xz
-        retention-days: 1
+        retention-days: 7
         compression-level: 0
 
     - name: Upload buildroot package directory