Added functionality to set flag_value correctly when using envvar

[chips-wq]

@click.option('--upper', flag_value='upper', envvar='UPPER')
def cmd(upper):
    click.echo(upper)
    return upper

Default use

The expectation of the above code when running python cmd.py --upper is to print to stdout upper (the flag value). This is default behaviour and works correctly.

Environment variable use

What is the expectation when we run UPPER=32 python cmd.py ?

The program should still print to the screen upper, because that is the flag_value and there exists an env variable defined with the name UPPER, yet it actually prints 32, the value of the environment variable, which i don't believe is intended.

I also added a test for this functionality called test_envvar_flag_value
fixes #2746