terminusdb: fix baked-in sandbox paths in Prolog saved state

pkgs/by-name/te/terminusdb/package.nix

@@ -100,6 +100,12 @@ stdenv.mkDerivation (finalAttrs: {
     substituteInPlace $cargoDepsCopy/*/tikv-jemalloc-sys-*/build.rs \
       --replace-fail 'format!("{orig_makeflags} {makeflags}")' \
                      'format!("{makeflags} {orig_makeflags}")'
+
+    # Hardcode terminus_home to a Nix store path that will exist both
+    # at compile time (via the symlink in preBuild) and at runtime.
+    substituteInPlace src/load_paths.pl \
+      --replace-fail "top_level_directory(TopDir)," \
+                     "TopDir = '$out/share/terminusdb',"
   '';
 
   strictDeps = true;
@@ -151,6 +157,11 @@ stdenv.mkDerivation (finalAttrs: {
   preBuild = ''
     export TERMINUSDB_GIT_HASH=$(cat $src/COMMIT)
     export TERMINUSDB_JWT_ENABLED=true
+
+    # Point terminus_home at the build directory during compilation
+    # so the Rust dylib and Prolog sources are findable.
+    mkdir -p $out/share/terminusdb
+    ln --symbolic --force --no-target-directory $PWD/src $out/share/terminusdb/src
   '';
 
   # Required for Prolog initialisation
@@ -160,6 +171,10 @@ stdenv.mkDerivation (finalAttrs: {
     runHook preInstall
     installBin terminusdb
     installManPage $src/docs/terminusdb.1
+
+    # Replace with the Nix store source so schema files are findable
+    # at runtime (the build directory no longer exists).
+    ln --symbolic --force --no-target-directory $src/src $out/share/terminusdb/src
     runHook postInstall
   '';