CI: skip some jobs when repo != containerd/containerd

For running CI in a non-upstream repo

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.github/workflows/fuzz.yml

@@ -8,6 +8,7 @@ jobs:
   # Others use https://github.com/AdaLogics/go-fuzz-headers.
   ci_fuzz:
     name: CI Fuzz
+    if: github.repository == 'containerd/containerd'
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -35,6 +36,7 @@ jobs:
   # runnable with go test -fuzz.
   go_test_fuzz:
     name : go test -fuzz
+    if: github.repository == 'containerd/containerd'
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps: