CI: allow the runner to set a NVD API key

Setting an API key means we get higher rate limits. Because keys are
private, the key must be set in the environment of the runner.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -19,6 +19,8 @@ variables:
   ACS_TEST: 0
   # The list of extra Kas fragments to be used when building
   EXTRA_KAS_FILES: ""
+  # The NVD API key to use when fetching CVEs
+  NVDCVE_API_KEY: ""
 
 stages:
   - prep