terraform: increase max concurrent dispatches by 2x

The hope is that this will speed up the pipeline by two times without
resulting in additional "unavailable container instances" errors.
Otherwise, this CL might be reverted (although the mentioned errors are
supposedly retried).

Change-Id: I777254ad6ab3b2c63fda576e001981a98649fc5a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/499276
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
diff --git a/terraform/environment/worker.tf b/terraform/environment/worker.tf
index 1d493f5..aff5627 100644
--- a/terraform/environment/worker.tf
+++ b/terraform/environment/worker.tf
@@ -213,7 +213,7 @@
   project  = var.project
 
   rate_limits {
-    max_concurrent_dispatches = 100
+    max_concurrent_dispatches = 200
     max_dispatches_per_second = 500
   }