I haven’t got the slightest idea if avast would detect this as a virus or suspect email or not. But as they say if you aren’t a crook or doing anything wrong then you probably have nothing to fear from the FBI or our own Police/Security services ;D
First if you read some of the articles that you posted links too it has to be installed.
Magic Lantern can reportedly be installed remotely, via an e-mail attachment or by exploiting common operating system vulnerabilities, unlike previous keystroke logger programs used by the FBI.[3][4] It has been variously described as a virus and a Trojan horse. It is not known how the program might store or communicate the recorded keystrokes.
So first you have to receive an email with this attachment and presumably this would be an unsolicited email not from a ‘known source’ so you can do yourself a favour in using a pre filter anti-spam and filter out unsolicited emails. Exercise a degree of common sense, don’t open attachments or click on links in unsolicited/suspect emails.
Or by not keeping your system up to date and having it exploit a vulnerability in your OS or other internet facing software. So keep your system up to date is a good start; not breaking the law or bringing yourself to the attention of the authorities, etc. etc. would help.