tf-a-commitlint: run `nvm use`

This change ensures that NVM is using a version of Node.js compatible
with the repository's Node.js tooling.

Change-Id: Iced22f9b9c185e171666875094a2f1d50020cc89
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/tf-a-commitlint/Jenkinsfile b/tf-a-commitlint/Jenkinsfile
index 3a9041a..be56694 100644
--- a/tf-a-commitlint/Jenkinsfile
+++ b/tf-a-commitlint/Jenkinsfile
@@ -44,6 +44,7 @@
 
         stage('Lint') {
             steps {
+                sh "BASH_ENV=$NVM_DIR/nvm.sh bash -c '[ -f .nvmrc ] && nvm use'"
                 sh "BASH_ENV=$NVM_DIR/nvm.sh bash -c 'npm install --no-save commitlint'"
                 sh "BASH_ENV=$NVM_DIR/nvm.sh bash -c 'npx commitlint --from=${mergeBase}'"
             }