Adjust generation of env file that starts with BUILDKITE word

[RafalKorepta]

The following 2 environment variables was added to each buildkite job

BASH_FUNC___buildkite_clean_unused_paths%%=() {  REDPANDA_BUILD_CHECKOUT_PATH=$(dirname "$BUILDKITE_BUILD_CHECKOUT_PATH");
 cd $REDPANDA_BUILD_CHECKOUT_PATH;
 for i in $(ls -1 . | grep -v "^${BUILDKITE_PIPELINE_NAME}\$");
 do
 echo "Cleaning $i";
 rm -rf ./$i;
 done;
 cd -
}
BASH_FUNC___buildkite_chown_checkout_path%%=() {  user_id=$UID;
 group_id=$(id -g);
 REDPANDA_BUILD_CHECKOUT_PATH=$(dirname "$BUILDKITE_BUILD_CHECKOUT_PATH");
 echo "Changing ownership of $REDPANDA_BUILD_CHECKOUT_PATH to '${user_id}:${group_id}'";
 docker run --rm --privileged --userns=host -t -v "$REDPANDA_BUILD_CHECKOUT_PATH":"$REDPANDA_BUILD_CHECKOUT_PATH" --workdir="$REDPANDA_BUILD_CHECKOUT_PATH" public.ecr.aws/docker/library/alpine:3 chown -R ${user_id}:${group_id} $REDPANDA_BUILD_CHECKOUT_PATH
}

As grep was looking for any BUILDKITE word 3 lines from the above functions was added to env-file-for-buildkite which was passed to docker run command.

[RafalKorepta]

I made mistake by merging #410

This PR will adjust the grep now.