CI - Fix caching in C#/Unity testsuite (#3194)

# Description of Changes

We had weird caching issues in the C#/Unity testsuite. Somehow, they got
triggered only as of
#3181 merging, and I
have no idea why/how.

I've restored the `id` field of the checkout step (which is used by the
cache step), and this _seems_ to have fixed it.

# API and ABI breaking changes

None.

# Expected complexity level and risk

1

# Testing

- [x] It passes on this PR
- [x] It passes in a test PR that combines this change with
#3182

---------

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Author: bfops

.github/workflows/csharp-test.yml

@@ -16,6 +16,7 @@ jobs:
     timeout-minutes: 30
     steps:
       - name: Checkout repository
+        id: checkout-stdb
         uses: actions/checkout@v4
 
       # Run cheap .NET tests first. If those fail, no need to run expensive Unity tests.