Actually pass cargo flags instead of literal $CARGO_FLAGS (#143)

As it turns out, this actually gets interpreted as a test filter and so
CI doesn't end up running any tests.

.github/workflows/build-project.yml

@@ -25,7 +25,7 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: test
-          args: --color always $CARGO_FLAGS
+          args: --color always ${{ env.CARGO_FLAGS }}
 
   format:
     name: check-fmt