jenkins: move retry+timeout out of groovy into shell

Jenkinsfile

@@ -220,14 +220,12 @@ def abortOnError(msg)
 
 def fetchPR(prNum, fetchArgs, extraRefSpec)
 {
-    retry(3) {
-        timeout(time: 60, unit: 'SECONDS') {
-            deleteDir()
-            sh """git init
-            if (( "\${RIOT_MIRROR}" )); then RIOT_URL="\${RIOT_MIRROR_URL}"; else RIOT_URL="https://github.com/RIOT-OS/RIOT"; fi
-            git remote add origin "\${RIOT_URL}"
-            git fetch -u -n ${fetchArgs} origin ${extraRefSpec} pull/${prNum}/merge:pull_${prNum}
-            git checkout pull_${prNum}"""
-        }
-    }
+    sh """git init
+    if (( "\${RIOT_MIRROR}" )); then RIOT_URL="\${RIOT_MIRROR_URL}"; else RIOT_URL="https://github.com/RIOT-OS/RIOT"; fi
+    git remote add origin "\${RIOT_URL}"
+    for RETRIES in {1..3}; do
+        timeout 30 git fetch -u -n ${fetchArgs} origin ${extraRefSpec} pull/${prNum}/merge:pull_${prNum} && break
+    done
+    [[ "\$RETRIES" -eq 3 ]] && exit 1
+    git checkout pull_${prNum}"""
 }