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/actions/go-setup/action.yml

@@ -14,6 +14,7 @@ runs:
   steps:
     - uses: ./.github/actions/free-disk-space
     - uses: actions/setup-go@b7ad1dad31e06c5925ef5d2fc7ad053ef454303e # v7
+      id: setup-go
       with:
         go-version-file: go.mod
         check-latest: true
@@ -21,4 +22,5 @@ runs:
     - if: ${{ inputs.cache == 'true' }}
       uses: ./.github/actions/go-cache
       with:
+        go-version: ${{ steps.setup-go.outputs.go-version }}
         lint-cache: ${{ inputs.lint-cache }}