[Android] Fix CI build script

Fix usage of BUILD_DIR variable that was missed during input parameters parsing

Signed-off-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>

android/buildAndroidBOINC-CI.sh

@@ -91,7 +91,7 @@ fi
 
 if [ "x$build_dir" != "x" ]; then
     if isPathCanonical "$build_dir" && [ "$build_dir" != "/" ]; then
-         PREFIX="$build_dir"
+         BUILD_DIR="$build_dir"
      else
          echo "build_dir must be an absolute path without ./ or ../ in it"
          exit 1