[AIRFLOW-4799] don't mutate self.env in BashOperator execute method (#5421)
[AIRFLOW-4799] don't mutate self.env in BashOperator execute method (#5421)

* in tests using bash operator repeatedly, env is populated with contents of environment.
* on subsequent runs, render_templates will try to render contents of env.
* this produces unpredictable behavior where missing template error may be thrown, or env paths may be replaced with "template file" contents
apache/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,DinoCow/airflow,cfei18/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,Fokko/incubator-airflow,airbnb/airflow,mistercrunch/airflow,wooga/airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,apache/airflow,nathanielvarona/airflow,mrkm4ntr/incubator-airflow,mrkm4ntr/incubator-airflow,spektom/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,airbnb/airflow,Fokko/incubator-airflow,spektom/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,lyft/incubator-airflow,airbnb/airflow,dhuang/incubator-airflow,Fokko/incubator-airflow,DinoCow/airflow,mistercrunch/airflow,bolkedebruin/airflow,mtagle/airflow,apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,wileeam/airflow,spektom/incubator-airflow,DinoCow/airflow,DinoCow/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,mtagle/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,wileeam/airflow,sekikn/incubator-airflow,wooga/airflow,wooga/airflow,cfei18/incubator-airflow,apache/incubator-airflow,Fokko/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,mrkm4ntr/incubator-airflow,apache/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,wileeam/airflow,wileeam/airflow,spektom/incubator-airflow,mtagle/airflow,lyft/incubator-airflow,mtagle/airflow