Fix CI

Because we're building a library without an executable, 
running `nimble build` doesn't make sense.

.github/workflows/test.yml

@@ -12,6 +12,6 @@ jobs:
     - uses: actions/checkout@v2
     - uses: iffy/install-nim@v1.1
     - name: Build
-      run: nimble build -y
+      run: nimble install -y
     - name: Test
       run: nimble test -y