tf-a-commitlint: don't override `BASH_ENV`

The Dockerfile has been modified to ensure that this environment
variable is always set, and so NVM should always be available now.

Change-Id: I79407f21b63c32e9e87187ef4222816d092a58b1
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/tf-a-commitlint/Jenkinsfile b/tf-a-commitlint/Jenkinsfile
index 2cb5829..9d1d44f 100644
--- a/tf-a-commitlint/Jenkinsfile
+++ b/tf-a-commitlint/Jenkinsfile
@@ -46,12 +46,12 @@
             steps {
                 script {
                     if (fileExists('.nvmrc')) {
-                        sh "BASH_ENV=$NVM_DIR/nvm.sh bash -c 'nvm install'"
+                        sh "nvm install"
                     }
                 }
 
-                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=${mergeBase}'"
+                sh "npm install --no-save commitlint"
+                sh "npx commitlint --from=${mergeBase}"
             }
         }
     }