stdenv/freebsd: isolate bootstrap process more

Copying instead of symlinking prevents leakage of the bootstrap FHS
into the 0->1 bootstrap stage. The symptom this addresses is libssh2
failing because it can't find libssl, since it is failing to use the
ld wrapper.

pkgs/stdenv/freebsd/default.nix

···
         name = attrs.name or (builtins.baseNameOf (builtins.elemAt attrs.paths 0));
         src = bootstrapArchive;
         builder = "${bootstrapArchive}/bin/bash";
+        # this script will prefer to link files instead of copying them.
+        # this prevents clang in particular, but possibly others, from calling readlink(argv[0])
+        # and obtaining dependencies, ld(1) in particular, from there instead of $PATH.
         args = [ ./linkBootstrap.sh ];
         PATH = "${bootstrapArchive}/bin";
         paths = attrs.paths;

pkgs/stdenv/freebsd/linkBootstrap.sh

···
         exit 1
     fi
     mkdir -p $out/$(dirname $path)
-    ln -s $src/$path $out/$path
+    if [[ -d $src/$path ]]; then
+        ln -s $src/$path $out/$path
+    else
+        cp -RL $src/$path $out/$path
+    fi
 done