test: run coverage on pull requests

.github/workflows/coverage.yml

diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index e0e01a7..dc5b5a0 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -4,8 +4,7 @@
 name: "Coverage"
 
 on:
-  # As currently structured, this adds too many jobs (checks?), so don't run it
-  # on pull requests yet.
+  pull_request:
   push:
     branches:
       - main