ci: Report code coverage (#487)

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index f00ae85..941bcc1 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -111,7 +111,7 @@
         run: go build ./...
       -
         name: Run unit tests
-        run: go test -race $(go list ./... | grep -v /tfexec/internal/e2etest)
+        run: go test -cover -race $(go list ./... | grep -v /tfexec/internal/e2etest)
 
   e2e-tests:
     name: e2e-tests (${{ matrix.os }}, go ${{ matrix.go_version }}, terraform ${{ matrix.terraform_version }})