Update ci.yaml (#77)

Change CI to on: `pull_request`

Author: rcoh

.github/workflows/ci.yaml

@@ -1,4 +1,4 @@
-on: [ push ]
+on: [ pull_request ]
 
 env:
   rust_version: 1.51.0
@@ -24,4 +24,4 @@ jobs:
           toolchain: ${{ env.rust_version }}
       - name: Cargo Test
         run: cargo test
-        working-directory: sdk
+        working-directory: sdk