Fix cygwin build triplet hack

Setting a var this way doesn't actually change it for expansion in the same line..

diffutils/PKGBUILD

@@ -2,7 +2,7 @@
 
 pkgname=diffutils
 pkgver=3.8
-pkgrel=2
+pkgrel=3
 pkgdesc="Utility programs used for creating patch files"
 arch=('i686' 'x86_64')
 url="https://www.gnu.org/software/diffutils"
@@ -31,9 +31,9 @@ build() {
   # make sure tests are running OK
   sed -i "s/mkdir -m 0700/mkdir /" tests/init.sh
 
-  CHOST="${CHOST/-msys/-cygwin}" \
+  local CYGWIN_CHOST="${CHOST/-msys/-cygwin}"
   ./configure \
-    --build=${CHOST} \
+    --build=${CYGWIN_CHOST} \
     --prefix=/usr \
     --without-libiconv-prefix \
     --without-libintl-prefix