CI: skip Snyk check on forks PR (#3453)

Since the Snyk action requires some secrets that aren't not available on PRs created
from a fork, we skip them to avoid this kind of security issues.

Closes: #3451

Signed-off-by: Tao Li <tao.li@enterprisedb.com>
Signed-off-by: Jonathan Gonzalez V <jonathan.gonzalez@enterprisedb.com>
Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>
Co-authored-by: Jonathan Gonzalez V <jonathan.gonzalez@enterprisedb.com>
Co-authored-by: Niccolò Fei <niccolo.fei@enterprisedb.com>
(cherry picked from commit 057039a)

.github/workflows/continuous-integration.yml

@@ -560,6 +560,9 @@ jobs:
 
       - name: Run Snyk to check Docker image for vulnerabilities
         uses: snyk/actions/docker@master
+        if: |
+          github.repository_owner == 'cloudnative-pg' &&
+          !github.event.pull_request.head.repo.fork
         continue-on-error: true
         env:
           SNYK_TOKEN: ${{ secrets.SNYK_TOKEN }}
@@ -570,7 +573,8 @@ jobs:
       - name: Upload result to GitHub Code Scanning
         uses: github/codeql-action/upload-sarif@v2
         if: |
-          github.repository_owner == 'cloudnative-pg'
+          github.repository_owner == 'cloudnative-pg' &&
+          !github.event.pull_request.head.repo.fork
         with:
           sarif_file: snyk.sarif