Use savi deps update --for spec in library-check/spec CI job.

Without `--for spec` specified, it may be ambiguous to the Savi compiler
to know which particular manifest to pull in dependencies for.

Author: jemc

.github/workflows/library-check.yaml

@@ -25,7 +25,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: savi-lang/action-install@v1.0.0
-      - run: savi deps update
+      - run: savi deps update --for spec
       - run: savi run spec
 
   # Check formatting of all files in the repository.