Limit CI testing to relevant changes

This modification to the CI config should cause the test-build job
to be added to the pipeline only if changes are present which impact
the build of veilid-core/server/cli.

If successfull this will reduce the number of irrelevant test runs,
such as when .gitlab-ci.yml or instructional files are modified,
but still run Semgrep against all file changes.
[ci skip]

.gitlab-ci.yml

@@ -92,6 +92,9 @@ test_build:
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
     - if: $CI_PIPELINE_SOURCE == "push"
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+    - changes:
+        - veilid-*/**
+        - '**/*[Cc]argo*'
 
 semgrep:
   # A Docker image with Semgrep installed.