fix(ci): quote negated YAML expression to avoid tag parse error

Bare `!contains(...)` is interpreted as a YAML tag. Wrap in `\${{ }}`
to force expression evaluation.

Co-authored-by: Claude <noreply@anthropic.com>

Author: samantha-earthmover

.github/workflows/rust-ci.yaml

@@ -135,7 +135,7 @@ jobs:
           just profile=ci run-all-examples
 
       - name: Run unit tests only
-        if: !contains(matrix.os, 'linux') && !contains(matrix.os, 'ubuntu')
+        if: ${{ !contains(matrix.os, 'linux') && !contains(matrix.os, 'ubuntu') }}
         run: |
           cargo test --lib --profile ci