Add a build-network: True manifest option

[cgwalters]

Related: #48

This is an easy hack; I'm broadly speaking OK with relying on crates.io
for rpm-ostree's CI builds for now. If we take over and replace
the Koji stack, we can look at fixing the above issue.

[cgwalters]

Tested with this diff for FAHC:

$ git diff
diff --git a/overlay.yml b/overlay.yml
index 2432930..9af6df9 100644
--- a/overlay.yml
+++ b/overlay.yml
@@ -40,9 +40,8 @@ components:
   - src: github:projectatomic/rpm-ostree
     distgit:
       patches: drop
-    rpmwithout:
-      # https://github.com/projectatomic/rpmdistro-gitoverlay/issues/48
-      - rust
+    # https://github.com/projectatomic/rpmdistro-gitoverlay/issues/48
+    build-network: true
 
   - src: github:projectatomic/skopeo
     distgit:

[cgwalters]

/cc @jlebon too

[ashcrow]

👍 from me!

[ashcrow]

I'll wait for @jlebon before hitting the button.

[jlebon]

I'm not super familiar with this codebase, but it looks sane to me overall!