ci: improve caching (#38958)

- only `cache-seeder` writes caches, every other workflow restores.
Saves were being rejected once the repo went over its cache budget,
leaving main's caches stale and PR runs building cold
- seed the pnpm store and uv caches next to the go ones, so PRs
warm-start on them rather than installing from scratch
- prune keeps a single generation per key, including across go versions,
where a toolchain bump leaves the previous build cache unusable.
Reclaims ~2.6 GB immediately
- prune runs every 6h instead of daily and trims to 6 GB, since CodeQL
writes ~200 MB per push to main from outside this repo's workflows
- pull requests and release branches no longer write pnpm, uv and binfmt
caches, whose ref-scoped copies are never read again

---------

Signed-off-by: silverwind <me@silverwind.io>

.github/actions/go-cache/action.yml

@@ -31,7 +31,7 @@ runs:
       with:
         path: ~/go/pkg/mod
         key: gomod-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('go.sum') }}
-        restore-keys: gomod-${{ runner.os }}-${{ runner.arch }}
+        restore-keys: gomod-${{ runner.os }}-${{ runner.arch }}-
     - if: ${{ github.workflow == 'cache-seeder' && inputs.lint-cache != 'true' }}
       uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
       with: