fix: Produce prompts on stderr

[asottile]

This enables scriptability of aws-okta, this appears to already be the case for some of the prompting things.

before

(it hangs forever at the end of the output, I found it is prompting for the Role here)

$ ./aws-okta exec "$ROLE" -- echo hi > log
INFO[0001] Requesting MFA. Please complete two-factor authentication with your second device 
INFO[0001] Select a MFA from the following list         
INFO[0001] 0: DUO (web)                                 
INFO[0001] 1: GOOGLE (token:software:totp)              
0
INFO[0004] Sending Push Notification...                 
INFO[0005] Device: phone1           

after

(output is doctored a bit to obfuscate our account ids / etc.)

$ ./aws-okta exec "$ROLE" -- echo hi > log
INFO[0001] Requesting MFA. Please complete two-factor authentication with your second device 
INFO[0001] Select a MFA from the following list         
INFO[0001] 0: DUO (web)                                 
INFO[0001] 1: GOOGLE (token:software:totp)              
Select MFA method: 0

INFO[0003] Sending Push Notification...                 
INFO[0004] Device: phone1                               

Account: 1234
   0 - bless-user

<< snip >>

Select Role to Assume: 0
$

[nickatsegment]

Love it.

[asottile]

thanks!