buildFHSEnv: fix `NIX_LDFLAGS` propagation to `ld` wrapper

Before the change the following command did not work:

$ nix develop -i --impure --expr 'with import <nixpkgs> { system = "i686-linux"; }; (buildFHSUserEnv { name = "t"; targetPkgs = ps: with ps; [ libmpc stdenv.cc ]; }).env'
$ ld -lmpc -o a
ld: cannot find -lmpc: No such file or directory

It is expected to work as `NIX_LDFLAGS` does contain valid values:

$ echo $NIX_LDFLAGS
-L/usr/lib -L/usr/lib32

Note that for `gcc` it does work:

$ printf "int main(){}" | gcc -x c - -lmpc -o a

It happens because `HOST` role is enabled for `cc`:

$ echo $NIX_CC_WRAPPER_TARGET_HOST_i686_unknown_linux_gnu
1

But not for `BINTOOLS`:

$ echo $NIX_BINTOOLS_WRAPPER_TARGET_HOST_i686_unknown_linux_gnu
<empty>

The change adds BINTOOLS role and fixes linking:

$ nix develop -i --impure --expr 'with import ~/nm { system = "i686-linux"; }; (buildFHSUserEnv { name = "t"; targetPkgs = ps: with ps; [ libmpc stdenv.cc ]; }).env'
$ printf "int main(){}" | gcc -x c - -lmpc -o a
$ ld -lmpc -o a
ld: warning: cannot find entry symbol _start; not setting start address

pkgs/build-support/build-fhsenv-bubblewrap/buildFHSEnv.nix

···
 
     # Force compilers and other tools to look in default search paths
     unset NIX_ENFORCE_PURITY
+    export NIX_BINTOOLS_WRAPPER_TARGET_HOST_${stdenv.cc.suffixSalt}=1
     export NIX_CC_WRAPPER_TARGET_HOST_${stdenv.cc.suffixSalt}=1
     export NIX_CFLAGS_COMPILE='-idirafter /usr/include'
     export NIX_CFLAGS_LINK='-L/usr/lib -L/usr/lib32'

pkgs/build-support/build-fhsenv-chroot/env.nix

···
 
     # Force compilers and other tools to look in default search paths
     unset NIX_ENFORCE_PURITY
+    export NIX_BINTOOLS_WRAPPER_TARGET_HOST_${stdenv.cc.suffixSalt}=1
     export NIX_CC_WRAPPER_TARGET_HOST_${stdenv.cc.suffixSalt}=1
     export NIX_CFLAGS_COMPILE='-idirafter /usr/include'
     export NIX_CFLAGS_LINK='-L/usr/lib -L/usr/lib32'