When using "try-<platform>" trigger unit tests

There is currently no way to trigger unit tests using the bors-servo try
infrastructure. This change makes it so that using try-<platform>
triggers unit tests for that platform. This will be especially useful
when testing changes for Windows before trying to land them.

.github/workflows/mac.yml

@@ -66,7 +66,7 @@ jobs:
       - name: Script tests
         run: ./mach test-scripts
       - name: Unit tests
-        if: ${{ inputs.unit-tests }}
+        if: ${{ inputs.unit-tests || github.ref_name == 'try-mac' }}
         run: python3 ./mach test-unit --release --with-${{ env.LAYOUT }}
       - name: Package
         run: python3 ./mach package --release