[MMTk/CI] Skip Ractor btests with MMTk

currently these are flaky, so until we can make them more robust, we'll
skip them for MMTk CI

.github/workflows/modgc.yml

@@ -124,6 +124,7 @@ jobs:
           echo 'MMTK_PLAN=${{ matrix.gc.mmtk_plan }}' >> $GITHUB_ENV
           echo 'EXCLUDES=../src/test/.excludes-mmtk' >> $GITHUB_ENV
           echo 'MSPECOPT=-B../src/spec/mmtk.mspec' >> $GITHUB_ENV
+          echo 'GITHUB_WORKFLOW=ModGC' >> $GITHUB_ENV
         if: ${{ matrix.gc.name == 'mmtk' }}
 
       - run: $SETARCH make

bootstraptest/test_ractor.rb

@@ -215,7 +215,8 @@ assert_equal '[:a, :b, :c, :d, :e, :f, :g]', %q{
 ###
 # Ractor still has several memory corruption so skip huge number of tests
 if ENV['GITHUB_WORKFLOW'] &&
-   ENV['GITHUB_WORKFLOW'] == 'Compilations'
+   (ENV['GITHUB_WORKFLOW'] == 'Compilations' ||
+   ENV['GITHUB_WORKFLOW'] == 'ModGC')
    # ignore the follow
 else