Fix nix-portable pkgStatic symlink handling

Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

pkgs/nix-portable/default.nix

@@ -69,7 +69,13 @@ let
     { nativeBuildInputs = [ upx ]; }
     ''
       mkdir -p $out/bin
-      upx -9 -o $out/bin/${binName} ${binPath}
+      theBinPath=${binPath}
+
+      if [[ -L "$theBinPath" ]]; then
+        theBinPath=$(readlink -f "$theBinPath")
+      fi
+
+      upx -9 -o $out/bin/${binName} $theBinPath
     '';
 
   installBin = pkg: bin: ''