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/workflows/release-nightly.yml

@@ -80,6 +80,8 @@ jobs:
       # fetch all tags to ensure that "git describe" reports expected Gitea version, eg. v1.21.0-dev-1-g1234567
       - run: git fetch --unshallow --quiet --tags --force
       - uses: docker/setup-qemu-action@96fe6ef7f33517b61c61be40b68a1882f3264fb8 # v4.2.0
+        with:
+          cache-image: false
       - uses: docker/setup-buildx-action@bb05f3f5519dd87d3ba754cc423b652a5edd6d2c # v4.2.0
       - name: Get cleaned branch name
         id: clean_name