tf-m-release.yaml: Switch back to master branch

TF-M v1.8.0 release test has been finished. So switch the default
branches back to master branch for the regular verification.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I0a40eabd7c6b50ff259d94a2bcfa07b282c9cdd7
1 file changed