Clone repo over https

[philrz]

When submitting PR #58, the CI failed for Windows with this complaint:

$ git clone --depth=1 --no-checkout --single-branch git://github.com/brimdata/zeek
Cloning into 'zeek'...
fatal: remote error: 
  The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.

Just changing to https:// should make it work.

[mattnibs]

Huh there seems to be unrelated build failures.

[philrz]

I now have a fix for the build failure that I was able to verify using some personal forks. The tl;dr is that the fix is shown in philrz/actor-framework@63e1ade. Because of how Zeek uses multiple layers of submodules, wiring this up was a multi-step process. I had to:

1. Make a fork of https://github.com/zeek/actor-framework with a branch that has the fix atop the actor-framework at the point we've been using, then,
2. Make a similar fork+branch of https://github.com/zeek/broker that points to that branch commit in that actor-framework fork, then,
3. Point a Zeek fork at that branch commit in that broker fork.

I was effectively a submodule novice when I started hacking at this so I may not have pieced it together in the most elegant way, but it built successfully in the end. I'll post some steps to propose how to do it using the forks in the brimdata GitHub org.

[philrz]

I'm going to close this PR and open a new issue that walks through the steps of fixing the wider build issue, and will include this small git:// -> https:// change as part of that.