CI: build Terravalet

The CI was only running the lint and test targets but not building Terravalet.
I think this was because currently the release process is local (not in CI),
but in any case it makes sense to also build the executable in CI for two reasons:
- one more check that we don't have glaring errors
- now the build target checks that the repo doesn't contain lightweight tags

Part of PCI-2275

Author: marco-m-pix4d

.github/workflows/ci.yml

@@ -28,3 +28,4 @@ jobs:
     - run: task install:deps
     - run: task lint
     - run: task test
+    - run: task build