stdenv, swift: Use local fd in is* bash functions for hygiene

pkgs/development/compilers/swift/patches/build-script-pax.patch

@@ -1,12 +1,13 @@
 --- swift/utils/build-script-impl	2017-01-23 12:47:20.401326309 -0600
 +++ swift-pax/utils/build-script-impl	2017-01-23 13:24:10.339366996 -0600
-@@ -1823,6 +1823,16 @@ function set_lldb_xcodebuild_options() {
+@@ -1823,6 +1823,17 @@ function set_lldb_xcodebuild_options() {
      fi
  }
  
 +## XXX: Taken from nixpkgs /pkgs/stdenv/generic/setup.sh
 +isELF() {
 +    local fn="$1"
++    local fd
 +    local magic
 +    exec {fd}< "$fn"
 +    read -n 4 -u $fd magic

pkgs/stdenv/generic/setup.sh

@@ -194,6 +194,7 @@ _addRpathPrefix() {
 # Return success if the specified file is an ELF object.
 isELF() {
     local fn="$1"
+    local fd
     local magic
     exec {fd}< "$fn"
     read -n 4 -u $fd magic
@@ -205,6 +206,7 @@ isELF() {
 # "#!").
 isScript() {
     local fn="$1"
+    local fd
     local magic
     if ! [ -x /bin/sh ]; then return 0; fi
     exec {fd}< "$fn"