ci: realize walker src via nix build to prime cold store

Diagnosed the persistent "path '…-source' is not valid": it's walker's
flake reading its own source at eval time — importTOML
resources/config.toml in its home-manager module and lib.fileset.toSource
for the package src. On a cold store the walker input source isn't
materialized, and neither --no-build flake check nor `nix flake archive`
makes the fileset-derived src readable. Replace the archive step with a
`nix build` of walker's src attribute, which eagerly realizes it (and the
input it reads from) so the subsequent flake check / eval matrix find
every path valid.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/check.yml

@@ -70,15 +70,19 @@ jobs:
           curl -L "https://releases.nixos.org/nix/nix-${NIX_VERSION}/install" | sh -s -- --no-daemon
           echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
 
-      - name: Materialize flake inputs into the store
-        # Nix keeps flake-input sources in a git cache and only materialises
-        # the `-source` store paths lazily. Stylix/base16.nix do
-        # import-from-derivation (readFile out of those sources) during eval,
-        # which needs them as real store paths — on a cold CI store that
-        # fails with "path '…-source' is not valid" (works locally only
-        # because prior builds already materialised them). `flake archive`
-        # copies every transitive input into the store up front.
-        run: nix flake archive
+      - name: Materialize flake-input sources for eval
+        # Walker's flake reads its own source tree during evaluation:
+        # importTOML resources/config.toml in its home-manager module and
+        # lib.fileset.toSource for the package src (cargoLock.lockFile =
+        # "${src}/Cargo.lock"). On a cold store the input source isn't
+        # materialised, so flake check / eval die with "path '…-source' is
+        # not valid" (works locally only because prior builds materialised
+        # it; `nix flake archive` reports the input but doesn't make the
+        # fileset-derived src readable). `nix build` of walker's src
+        # eagerly realises it — and the input it reads from — priming the
+        # store so the later eval steps find every path valid.
+        run: |
+          nix build --no-link --impure --expr 'let f = builtins.getFlake (toString ./.); in f.inputs.walker.packages.${builtins.currentSystem}.default.src'
 
       - name: nix flake check --no-build
         run: nix flake check --no-build