tf-a-commitlint: ensure NPM calls are routed via NVM

The Docker image has Node.js v14 preinstalled, which NVM defaults to
using when a new shell is opened (which occurs on every `sh` step).

This change ensures that NPM is run via NVM, which will then pick up the
NPM version specified by the `.nvmrc` file.

Change-Id: I9d4e86f55c27f2183eaf864ff149b2a34c796b45
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed