Set COCOAPODS_PARALLEL_CODE_SIGN to false if it isn't set

This is required for `set -u` to work since this variable isn't required
to be set.

lib/cocoapods/generator/embed_frameworks_script.rb

@@ -42,6 +42,7 @@ module Pod
           echo "mkdir -p ${CONFIGURATION_BUILD_DIR}/${FRAMEWORKS_FOLDER_PATH}"
           mkdir -p "${CONFIGURATION_BUILD_DIR}/${FRAMEWORKS_FOLDER_PATH}"
 
+          COCOAPODS_PARALLEL_CODE_SIGN="${COCOAPODS_PARALLEL_CODE_SIGN:-false}"
           SWIFT_STDLIB_PATH="${DT_TOOLCHAIN_DIR}/usr/lib/swift/${PLATFORM_NAME}"
 
           # Used as a return value for each invocation of `strip_invalid_archs` function.