static-nix-shell: more clearly mark the nix-shell preambles

pkgs/by-name/static-nix-shell/package.nix

@@ -105,13 +105,19 @@ in rec {
 
       buildPhase = ''
         runHook preBuild
-        #^ TODO: each shell should use preBuild to compute a `shellPreamble`
 
         die() {
           echo "$@"
           exit 1
         }
 
+        if [ -n "$shellPreamble" ]; then
+        shellPreamble="
+        # --- BEGIN: nix-shell preamble (generated code) ---
+        $shellPreamble
+        # --- END:   nix-shell preamble (generated code) ---
+        "
+        fi
         substituteInPlace ${srcPath} \
           --replace-fail '#!/usr/bin/env nix-shell' '#!${interpreter}' \
           --replace-fail \
@@ -241,6 +247,7 @@ in rec {
       pkgsEnv = [ zsh ] ++ (builtins.attrValues pkgsAsAttrs);
       pkgExprs = insertTopo "zsh" (builtins.attrNames pkgsAsAttrs);
     in mkShell ({
+      # TODO: generate `shellPreamble` here to reduce wrapping
       inherit pkgsEnv pkgExprs;
       interpreter = lib.getExe zsh;
     } // (removeAttrs attrs [ "pkgs" "zsh" ])
@@ -253,6 +260,7 @@ in rec {
       pkgsEnv = builtins.attrValues pkgsAsAttrs;
       pkgExprs = insertTopo "python3" (builtins.attrNames pkgsAsAttrs);
     in mkShell ({
+      # TODO: generate `shellPreamble` here to reduce wrapping
       inherit pkgsEnv pkgExprs;
       interpreter = lib.getExe python3;
       interpreterName = "python3";