ci: improve go caches (#38730)

Cache saves have been rejected since June because the repo sat above its
10 GB allowance, so every PR run fell back to a cache built with an
older toolchain and rebuilt the backend from scratch.

- go version in the `gobuild` and `golint` keys and `restore-keys`
- seeder triggers on `go.mod` and gained `workflow_dispatch`
- single writer for the `gomod` cache, the two were racing
- new daily `cache-prune` workflow holding the total under a limit
- `cache: false` for `setup-go` in release and cron workflows, it held
815 MB
- add `workflow_dispatch` so the cache workflows can be triggered
on-demand

Assisted-by: Claude Code:claude-opus-5

.github/workflows/release-nightly.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           go-version-file: go.mod
           check-latest: true
+          cache: false
       - uses: ./.github/actions/node-setup
       - run: make deps-frontend deps-backend
       # xgo build