Reduce GitHub API usage by trusting nobody

Checking for collaborator permissions requires GitHub API calls, and we
never change the Jenkinsfile in the repos, so fetch the upstream
Jenkinsfile unconditionally.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: Idfee2992c899925fda0b85b191dd501ef62031c7
diff --git a/mbed-tls-pr-merge.yaml b/mbed-tls-pr-merge.yaml
index a9fd447..692038f 100644
--- a/mbed-tls-pr-merge.yaml
+++ b/mbed-tls-pr-merge.yaml
@@ -36,7 +36,7 @@
         repo: mbedtls
         branch-discovery: no-pr
         discover-pr-forks-strategy: both
-        discover-pr-forks-trust: permission
+        discover-pr-forks-trust: nobody
         discover-pr-origin: both
         head-filter-regex: '(?!PR-7959-merge$)PR-\d+-merge'
         status-checks: