Exclude sheepdog in nextest runs (#1631)

### What does this PR do?

In PR #1590 I attempted to move sheepdog to CI-only integration
    tests. However I failed to realize that sheepdog tests would still run
    as a part of nextest runs because of the way workspaces are arranged
    in this project. This commit should make local `ci/validate` loops
    much faster.

Author: blt

ci/test

@@ -10,6 +10,6 @@ if ! command -v cargo-nextest &> /dev/null; then
 fi
 
 echo "Running cargo nextest..."
-cargo nextest run --all-features "$@"
+cargo nextest run --workspace --all-features --exclude sheepdog "$@"
 
 echo "Tests passed!"