ci: pass --locked to the build and test commands

This will prevent Cargo.toml changes from landing without the Cargo.lock
being up to date.

Author: pietroalbini

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
           echo "::set-env name=CRATESFYI_DATABASE_URL::postgresql://$(whoami)@%2Fvar%2Frun%2Fpostgresql/$(whoami)"
 
       - name: Build docs.rs
-        run: cargo build
+        run: cargo build --locked
 
       - name: Prepare the test environment
         run: |
@@ -37,7 +37,7 @@ jobs:
           echo "::set-env name=CRATESFYI_PREFIX::${prefix}"
 
       - name: Test docs.rs
-        run: cargo test -- --test-threads=1
+        run: cargo test --locked -- --test-threads=1
 
   docker:
     name: Docker