tf-m-coverity: Keep using CI master branch

CI scripts does not switch to "main" branch. So keep using "master"
branch by default.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I546ff4357453f180f5e2eb0c5bb3fc36e5243d58
diff --git a/tf-m-coverity.yaml b/tf-m-coverity.yaml
index 6b20d80..e1b8385 100644
--- a/tf-m-coverity.yaml
+++ b/tf-m-coverity.yaml
@@ -46,7 +46,7 @@
             default: 'https://git.trustedfirmware.org/ci/tf-m-ci-scripts'
         - string:
             name: CI_SCRIPTS_BRANCH
-            default: 'main'
+            default: 'master'
         - string:
             name: MBEDTLS_VERSION
             default: ''