CI: nix flake check runs all tests; Gitea workflow

- flake: checks.tests = the sausage package with doCheck (tests of sausage,
  sausage-actor, agent; fixtures build offline from vendored crates).
- .gitea/workflows/ci.yml: `nix flake check -L` on runner label `nixos`.

Not compiled: written by the Sausage agent.

flake.nix

@@ -58,6 +58,19 @@
       {
         packages.default = sausage;
 
+        # `nix flake check` runs every test of the host, the SDK, and the
+        # agent. The tests build WASM actors with cargo, offline, from the
+        # vendored crates of Cargo.lock.
+        checks.tests = sausage.overrideAttrs (old: {
+          pname = "sausage-tests";
+          doCheck = true;
+          cargoTestFlags = [ "-p" "sausage" "-p" "sausage-actor" "-p" "agent" ];
+          preCheck = ''
+            # Its own lock file pins crates that the vendored set may lack.
+            rm -f cli/tests/actors/toolbox/Cargo.lock
+          '';
+        });
+
         apps.default = { type = "app"; program = "${sausage}/bin/sausage"; };
         apps.loop = { type = "app"; program = "${loop "sausage-loop" publicUrl}/bin/sausage-loop"; };
         apps.loop-internal = { type = "app"; program = "${loop "sausage-loop-internal" internalUrl}/bin/sausage-loop-internal"; };