fix: limit EXLA C++ compilation parallelism in CI to prevent OOM

g++ gets OOM-killed during EXLA NIF compilation in the test step.
MAKEFLAGS=-j1 reduces peak memory by compiling one translation unit
at a time.

.forgejo/workflows/build.yaml

@@ -92,6 +92,7 @@ jobs:
               -cf - . | \
           docker run --rm -i \
             -e MIX_ENV=test \
+            -e MAKEFLAGS="-j1" \
             --network host \
             -w /app \
             docker.io/hexpm/elixir:1.20.1-erlang-29.0.2-debian-trixie-20260518-slim \