Fix new buildkit builds on MacOS (#20963)
Fix new buildkit builds on MacOS (#20963)

MacOS requires array variables to be declared on top level
(Bash3).

This PR changes BUILD_COMMAND variable to be declared at top level.
mistercrunch/airflow,mistercrunch/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,apache/airflow,nathanielvarona/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,apache/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,apache/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,apache/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,nathanielvarona/airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,apache/airflow