ci/weekly: Only run scheduled jobs on the upstream repository (#435)

With the workflow being to create a fork in order to contribute to this
project, this fork shouldn't automatically run scheduled linting jobs on
the main branch (wherever that happened to be when the fork was
cloned/created) and bothering the fork-er with it if it's non-compliant.

Author: MarijnS95

.github/workflows/weekly.yml

@@ -11,6 +11,7 @@ jobs:
   weekly-clippy-beta:
     name: Clippy Beta
     runs-on: ubuntu-latest
+    if: github.repository_owner == 'getsentry'
 
     steps:
       - uses: actions/checkout@v2
@@ -35,6 +36,7 @@ jobs:
   weekly-audit:
     name: Audit
     runs-on: ubuntu-latest
+    if: github.repository_owner == 'getsentry'
 
     steps:
       - uses: actions/checkout@v2