attempt at fixing library paths

.tangled/workflows/build.yml

···
     command: |
       set -e
       eval $(opam env --switch 5.2.0+ox)
+      # Point C toolchain at nix-provided libraries
+      NIX_PROFILE="/tangled/home/.nix-profile"
+      export PKG_CONFIG_PATH="${NIX_PROFILE}/lib/pkgconfig:${PKG_CONFIG_PATH:-}"
+      export C_INCLUDE_PATH="${NIX_PROFILE}/include:${C_INCLUDE_PATH:-}"
+      export LIBRARY_PATH="${NIX_PROFILE}/lib:${LIBRARY_PATH:-}"
       opam install --deps-only -y .
 
   - name: "Build and test"