ci: fix installation condition for MinGW

I mistakenly inverted the "variable is not set" check in bash.

Author: pietroalbini

src/ci/scripts/install-mingw.sh

@@ -29,17 +29,17 @@ source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 
 if isWindows; then
     if [[ -z "${MINGW_URL+x}" ]]; then
-        curl -o mingw.7z "${MINGW_URL}/${MINGW_ARCHIVE}"
-        7z x -y mingw.7z > /dev/null
-        curl -o "${MINGW_DIR}/bin/gdborig.exe" "${MINGW_URL}/2017-04-20-${MSYS_BITS}bit-gdborig.exe"
-        ciCommandAddPath "$(pwd)/${MINGW_DIR}/bin"
-    else
         arch=i686
         if [ "$MSYS_BITS" = "64" ]; then
           arch=x86_64
         fi
         pacman -S --noconfirm --needed mingw-w64-$arch-toolchain mingw-w64-$arch-cmake \
             mingw-w64-$arch-gcc mingw-w64-$arch-python2
         ciCommandAddPath "${SYSTEM_WORKFOLDER}/msys2/mingw${MSYS_BITS}/bin"
+    else
+        curl -o mingw.7z "${MINGW_URL}/${MINGW_ARCHIVE}"
+        7z x -y mingw.7z > /dev/null
+        curl -o "${MINGW_DIR}/bin/gdborig.exe" "${MINGW_URL}/2017-04-20-${MSYS_BITS}bit-gdborig.exe"
+        ciCommandAddPath "$(pwd)/${MINGW_DIR}/bin"
     fi
 fi