bionic-amd64-tf-m-build: Update version of CppCheck

CppCheck v2.3 has a bug to report unexpected error of bad macro syntax.
This bug has been fixed in the latest version v2.6.
Update CppCheck to v2.6 on TF-M Open CI.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Icd87d8c781b9879fa198a0b2f8f8aaae5c79f171
1 file changed