[ci/nix-shell] uniquify NIX_LDFLAGS{_FOR_TARGET}

(cherry picked from commit 07b1af03)

.gitlab/shell.nix

@@ -14,7 +14,17 @@
   # the real fix is to teach terminfo to use libcurses on macOS.
   CONFIGURE_ARGS = "--with-intree-gmp --with-curses-libraries=${pkgs.ncurses.out}/lib";
 
-  buildInputs = (with pkgs; [
+  # magic speedup pony :facepalm:
+  #
+  # nix has the ugly habbit of duplicating ld flags more than necessary.  This
+  # somewhat consolidates this.
+  shellHook = ''
+  export NIX_LDFLAGS=$(for a in $NIX_LDFLAGS; do echo $a; done |sort|uniq|xargs)
+  export NIX_LDFLAGS_FOR_TARGET=$(for a in $NIX_LDFLAGS_FOR_TARGET; do echo $a; done |sort|uniq|xargs)
+  export NIX_LDFLAGS_FOR_TARGET=$(comm -3 <(for l in $NIX_LDFLAGS_FOR_TARGET; do echo $l; done) <(for l in $NIX_LDFLAGS; do echo $l; done))
+  '';
+
+  nativeBuildInputs = (with pkgs; [
     haskell.compiler.${compiler}
     haskell.packages.${compiler}.cabal-install
     haskell.packages.${compiler}.alex