I haven't been able to reproduce with the CLI or the embedded server.

Which SAPIs? (Apache module, FastCGI, etc.)
Does it happen if you reference $_SERVER at all and not just with ['argv'], as in

function notcalled() {
  $_SERVER;
}

Huh, yeah. If I just reference $_SERVER it will populate the return of getenv() with 'argv' and 'argc'.

I'm running an nginx / php-fpm (fpm-fcgi?) setup here.

So I did some more digging and this behavior is caused by php-fpm specifically: PHP core has an implementation to list environment variables as an array, and php-fpm is necessarily overriding it. What it does is copy $_ENV if defined, or else $_SERVER if defined, or else fall back to the original implementation plus some FastCGI values.
https://github.com/php/php-src/blob/PHP-7.1.12/sapi/fpm/fpm/fpm_main.c#L563

The ironic thing is that this started with request #69359 which wanted getenv to return an array because $_ENV is not always accessible and because it won't reflect changes made during runtime, but php-fpm's implementation (written years before when it was first introduced) does the opposite of that.

So you're getting argc/v because referencing $_SERVER instructs PHP to create that variable (see the auto_globals_jit setting) which naturally includes those two, and php-fpm is giving you a copy of that array.

I don't know why php-fpm wants to copy $_ENV/SERVER? Without knowing the reason for that, or exactly where else it gets used, I would think those two bits should be removed entirely...