do not purge cache as it errors

.gitea/workflows/build.yaml

@@ -20,14 +20,6 @@ jobs:
           # collect garbage until Nix store size (in bytes) is at most this number
           # before trying to save a new cache
           gc-max-store-size-linux: 1073741824
-          # do purge caches
-          purge: true
-          # purge all versions of the cache
-          purge-prefixes: cache-${{ runner.os }}-
-          # created more than this number of seconds ago relative to the start of the `Post Restore` phase
-          purge-created: 0
-          # except the version with the `primary-key`, if it exists
-          purge-primary-key: never
       - name: Metadata
         run: |
           nix flake metadata