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/cache-seeder.yml

@@ -8,10 +8,12 @@
 name: cache-seeder
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main
     paths:
+      - "go.mod" # a toolchain bump invalidates the build caches
       - "go.sum"
       - ".golangci.yml"
       - ".github/actions/go-cache/action.yml"
@@ -70,3 +72,10 @@ jobs:
       - run: make ${{ matrix.target }}
         env:
           TAGS: ${{ matrix.tags }}
+
+  # reclaims the caches this run superseded, so the next save still fits in the allowance
+  prune:
+    needs: [gobuild, lint]
+    permissions:
+      actions: write
+    uses: ./.github/workflows/cache-prune.yml