Properly determine paths when cross-compiling

pkgs/nix-portable/default.nix

@@ -19,6 +19,16 @@ with builtins;
   compression ? "zstd -19 -T0",
   lib ? pkgs.lib,
   pkgs ? import <nixpkgs> {},
+
+  # pkgs set from the host system (just for eval in installDynamic)
+  pkgsHost ?
+    if stdenv.hostPlatform == stdenv.buildPlatform then
+      pkgs
+    else
+      import pkgs.path {
+        system = pkgs.stdenv.hostPlatform.system;
+      },
+
   # hardcode executable to run. Useful when creating a bundle.
   bundledPackage ? null,
 
@@ -86,9 +96,9 @@ let
   '';
 
   installDynamic = pkgname: let
-    out = pkgs.${pkgname}.out;
+    out = builtins.unsafeDiscardStringContext pkgsHost.${pkgname}.out.outPath;
   in ''
-    if [ ! -e \$store${lib.removePrefix "/nix/store" pkgs.${pkgname}.out} ] ; then
+    if [ ! -e \$store${lib.removePrefix "/nix/store" out} ] ; then
       debug "Installing ${pkgname}"
       \$run \$store${lib.removePrefix "/nix/store" nix}/bin/nix build --impure --no-link --expr "
         (import ${nixpkgsSrc} {}).${pkgname}.out
@@ -254,7 +264,7 @@ let
     fi
 
     # Override $SHELL with nix bashInteractive
-    export SHELL="${pkgs.bashInteractive.out}/bin/bash"
+    export SHELL="${builtins.unsafeDiscardStringContext pkgsHost.bashInteractive.out.outPath}/bin/bash"
     export PS1="\n\[\033[1;32m\][\[\e]0;\u@\h: \w\a\]\u@\h:\w]\$\[\033[0m\] "
 
     # unset bash function aliases