Copy source tree to non-nix-builder build directory and cd to the right spot

builder.nix

@@ -23,12 +23,16 @@ let
          then imported.value
          else {
            src = filterSource
-             (path: type: type != "directory" || (baseNameOf path != ".git" && (all (x: path != x) filter)))
+             (path: type: baseNameOf path != ".git" && baseNameOf path != ".envrc" && (all (x: path != x) filter))
              (/. + root);
-           builder = ''
+           builder = let
+             relPath = dirOf (removePrefix (root + "/") builder);
+           in ''
              #!/bin/sh
              set -e
              pout=$(mktemp -u -p $PWD)
+             cp -r --reflink=auto --no-preserve=mode "$src/" src
+             cd src/${relPath}
              ${/. + builder} $d1 $d2 $out >$pout
              if [ -e $out -a -s $pout ]; then
                printf "Error: %s wrote to stdout and created \$3\n" "$d1" >&2