feat: use save-always in cache action (#5010)

Follows up on
https://github.com/leanprover/lean4/pull/5003#issuecomment-2284813940

.github/workflows/ci.yml

@@ -337,6 +337,7 @@ jobs:
           # fall back to (latest) previous cache
           restore-keys: |
             ${{ matrix.name }}-build-v3
+          save-always: true
       # open nix-shell once for initial setup
       - name: Setup
         run: |

.github/workflows/nix-ci.yml

@@ -62,6 +62,7 @@ jobs:
           # fall back to (latest) previous cache
           restore-keys: |
             ${{ matrix.name }}-nix-store-cache
+          save-always: true
       - name: Further Set Up Nix Cache
         shell: bash -euxo pipefail {0}
         run: |
@@ -85,6 +86,7 @@ jobs:
           # fall back to (latest) previous cache
           restore-keys: |
             ${{ matrix.name }}-nix-ccache
+          save-always: true
       - name: Further Set Up CCache Cache
         run: |
           sudo chown -R root:nixbld /nix/var/cache