fix(ci): cap cargo to two parallel jobs so concurrent test-binary links fit runner memory

Runs 514/516/519 (incl. master itself) died on 'ld terminated with
signal 9' — the workspace now links enough large test binaries that
per-core parallel linking OOMs the runner even at line-tables-only
debuginfo. CARGO_BUILD_JOBS=2 keeps at most two links in flight.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -35,8 +35,15 @@ jobs:
     # runner's memory — ld was OOM-killed (signal 9) on runs 447 and 486.
     # line-tables-only keeps file:line in panic backtraces while cutting the
     # link's memory footprint enough to fit the runner.
+    #
+    # CARGO_BUILD_JOBS=2: with one job per core, cargo links several large
+    # test binaries concurrently; as the workspace grew (runs 514/516/519)
+    # two+ simultaneous ld processes OOM-killed the runner again even at
+    # line-tables-only. Two jobs keeps at most two links in flight — the
+    # compile-throughput cost is small next to the cache-warm build.
     env:
       CARGO_PROFILE_DEV_DEBUG: line-tables-only
+      CARGO_BUILD_JOBS: '2'
 
     steps:
       - name: Checkout