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/python-setup/action.yml

@@ -0,0 +1,10 @@
+name: python-setup
+description: Set up uv and python and restore the uv cache
+
+runs:
+  using: composite
+  steps:
+    - uses: astral-sh/setup-uv@ae62891fec2bb8e7d6c99fc78c9fec3a63790f8d # v10.0.0
+      with:
+        python-version: 3.14
+        save-cache: ${{ github.workflow == 'cache-seeder' }}