**Apache Airflow version**: 1.10.15


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):
`Client Version: version.Info{Major:"1", Minor:"14", GitVersion:"v1.14.6", GitCommit:"96fac5cd13a5dc064f7d9f4f23030a6aeface6cc", GitTreeState:"clean", BuildDate:"2019-08-19T11:13:49Z", GoVersion:"go1.12.9", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"18+", GitVersion:"v1.18.16-eks-7737de", GitCommit:"7737de131e58a68dda49cdd0ad821b4cb3665ae8", GitTreeState:"clean", BuildDate:"2021-03-10T21:33:25Z", GoVersion:"go1.13.15", Compiler:"gc", Platform:"linux/amd64"}`
**Environment**:

- **Cloud provider or hardware configuration**: EKS
- **OS** (e.g. from /etc/os-release): Debian GNU/Linux 10 (buster)
- **Kernel** (e.g. `uname -a`): Linux airflow-web-84bf9f8865-s5xxg 4.14.209-160.335.amzn2.x86_64 #1 SMP Wed Dec 2 23:31:46 UTC 2020 x86_64 
- **Install tools**: helm (fork of the official chart)
- **Others**:
**What happened**:
We started using `env` variable in the built-in bash_operator. the goal was to add a single variable to be used as part of the command. but once using the variable, it caused all of the other environment variables to be ignored.

**What you expected to happen**:
we expected that any environment variable we are adding to this operator will be added or updated.
the expectation are wrong from this variable.
**How to reproduce it**:

`import os
os.environ["foo"] = "bar"
from datetime import datetime
from airflow import DAG
from airflow.models import TaskInstance
from airflow.operators.bash_operator import BashOperator

dag = DAG(dag_id='anydag', start_date=datetime.now())

#unsuccessfull example:
task = BashOperator(bash_command='if [ -z "$foo" ]; then exit 1; fi',env= {"foo1" : "bar1"},dag=dag, task_id='test')
ti = TaskInstance(task=task, execution_date=datetime.now())
result = task.execute(ti.get_template_context())


#successfull example:
task = BashOperator(bash_command='if [ -z "$foo" ]; then exit 1; fi',dag=dag, task_id='test1')
ti = TaskInstance(task=task, execution_date=datetime.now())
result = task.execute(ti.get_template_context())`

**Anything else we need to know**:
this happens every time it runs.

tests/operators/test_bash.py
#
#
#
import unittest
from datetime import datetime, timedelta
from tempfile import NamedTemporaryFile, TemporaryDirectory
from unittest import mock
import pytest
from parameterized import parameterized
from airflow.exceptions import AirflowException, AirflowSkipException
from airflow.models import DagRun
from airflow.models.dag import DAG
from airflow.operators.bash import BashOperator
from airflow.utils import timezone
from airflow.utils.state import State
from airflow.utils.types import DagRunType
DEFAULT_DATE = datetime(2016, 1, 1, tzinfo=timezone.utc)
END_DATE = datetime(2016, 1, 2, tzinfo=timezone.utc)
INTERVAL = timedelta(hours=12)
class TestBashOperator(unittest.TestCase):
    def test_echo_env_variables(self):