Move conditional setup logic from bash to actions (#3313)

Instead of checking for a setup command in bash, we use GitHub Actions
to determine if there is anything to run

### Motivation:

Slightly cleaner output when looking at logs.

### Modifications:

Moved an if statement from bash to yaml.

### Result:

The setup commands step will generally be skipped instead of executing a
no-op

Co-authored-by: Cory Benfield <lukasa@apple.com>

Author: madrob

.github/workflows/macos_tests.yml

@@ -312,10 +312,8 @@ jobs:
           persist-credentials: false
           submodules: true
       - name: Setup command
-        run: |
-          if [ -n "${{ matrix.config.setup_command }}" ]; then
-            bash -c "${{ matrix.config.setup_command }}"
-          fi
+        if: ${{ matrix.config.setup_command != '' }}
+        run: bash -c "${{ matrix.config.setup_command }}"
       - name: Swift build
         run: |
           if [ -n "${{ matrix.config.build_arguments_override }}" ]; then