.github/workflows: Restrict some workflows to the official repository

Prevent that the Coverity and send-email workflows fail for submitters
of pull requests.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

.github/workflows/coverity.yml

@@ -7,6 +7,7 @@ on:
 
 jobs:
   coverity:
+    if: github.repository == 'SCST-project/scst'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Project

.github/workflows/mail_notification.yml

@@ -10,6 +10,7 @@ on:
 jobs:
   send_mails:
     name: ${{matrix.commit.message}}
+    if: github.repository == 'SCST-project/scst'
     runs-on: ubuntu-latest
     strategy:
       max-parallel: 1