Use -j$(nproc) instead of -j3 in CI make builds

Replace hardcoded -j3 with -j$(nproc) in ci.yml, nightly.yml, and
release.yml to utilize all available cores on GitHub Actions runners.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/release.yml

@@ -279,9 +279,9 @@ jobs:
           eval $(opam config env)
           python scripts/mk_make.py --ml
           cd build
-          make -j3
-          make -j3 examples
-          make -j3 test-z3
+          make -j$(nproc)
+          make -j$(nproc) examples
+          make -j$(nproc) test-z3
           cd ..
       
       - name: Generate documentation