ci: fit the whole Nix store in the GitHub cache

The GitHub job's store is ~7.3 GiB but gc-max-store-size-linux is 6G, and
nothing roots the devshells, so cache-nix-action's pre-save `nix store gc`
deletes whichever paths cross the cap before every save. The snapshots land
missing the JDK and the Android SDK components, so both `nix develop` steps
re-download or rebuild them on every run.

Raise the cap to 10G, which clears the store, and bump the key to nix-v3-. The
first run after that downloads the full closure once and saves a complete
snapshot; the gutted nix-v2- snapshots are not restored, so there is one cold
run per ref and v3 is warm from then on.

Compressed at ~0.47x its store the snapshot is ~4.7 GiB at worst, inside the
10 GB per-repo budget. What pushes the repo over that budget today is the four
stale gradle- (v1) entries, which no restore-key can reach and which age out by
LRU.

.github/actions/nix/action.yml

@@ -9,6 +9,6 @@ runs:
         extra_nix_config: experimental-features = nix-command flakes
     - uses: nix-community/cache-nix-action@7df957e333c1e5da7721f60227dbba6d06080569 # v7.0.2
       with:
-        primary-key: nix-v2-${{ hashFiles('flake.lock', '**/*.nix') }}
-        restore-prefixes-first-match: nix-v2-
-        gc-max-store-size-linux: 6G
+        primary-key: nix-v3-${{ hashFiles('flake.lock', '**/*.nix') }}
+        restore-prefixes-first-match: nix-v3-
+        gc-max-store-size-linux: 10G