dist: tools: pr_check: adapt to RIOT CI

dist/tools/pr_check/check_labels.sh

@@ -22,7 +22,11 @@ else
     exit 2
 fi
 
-LABELS_JSON=$(${GET} "${GITHUB_API_HOST}/repos/${GITHUB_REPO}/issues/${TRAVIS_PULL_REQUEST}/labels" 2> /dev/null)
+if [ -n "$TRAVIS_PULL_REQUEST" ]; then
+    LABELS_JSON=$(${GET} "${GITHUB_API_HOST}/repos/${GITHUB_REPO}/issues/${TRAVIS_PULL_REQUEST}/labels" 2> /dev/null)
+elif [ -n "$CI_PULL_LABELS" ]; then
+    LABELS_JSON="$CI_PULL_LABELS"
+fi
 
 check_gh_label() {
     LABEL="${1}"

dist/tools/pr_check/pr_check.sh

@@ -33,7 +33,7 @@ if [ -n "${SQUASH_COMMITS}" ]; then
     EXIT_CODE=1
 fi
 
-if [ -n "$TRAVIS_PULL_REQUEST" ]; then
+if [ -n "$TRAVIS_PULL_REQUEST" -o -n "$CI_PULL_NR" ]; then
     if check_gh_label "NEEDS SQUASHING"; then
         echo -e "${CERROR}Pull request needs squashing according to its labels set on GitHub${CRESET}"
         EXIT_CODE=1