Increase concurrent blob and recipe downloads from registry

[kylewlacy]

Related to #147

Pretty self-explanatory: this PR bumps the total number of concurrent blob downloads from 10 to 200, and the total number of concurrent recipe downloads from 25 to 200.

I tested this in the packages repo from 4f8fd12 with this patch applied:

diff --git a/packages/std/toolchain/native/index.bri b/packages/std/toolchain/native/index.bri
index 174c2dc..6e814b9 100644
--- a/packages/std/toolchain/native/index.bri
+++ b/packages/std/toolchain/native/index.bri
@@ -440,6 +440,7 @@ function makePkgConfigPathsRelative(
 function syncTarball(
   recipe: std.AsyncRecipe<std.Directory>,
 ): std.Recipe<std.Directory> {
+  return std.recipe(recipe);
   recipe = std.collectReferences(std.directory({ recipe }));
 
   const tarredRecipe = std

Here's the total build times I got before and after this PR, starting with a totally empty Brioche dir each time:

	Total build time
Before	7m 9s
After	1m35s

[kylewlacy]

I'm opening this as a draft because I'd definitely want to do testing with values besides just 200 blobs / 200 recipes concurrently. Plus, I have some ideas for a larger change that might make this PR irrelevant, but nothing ready to test yet.