CI: Rework qemuarm64-secureboot matrix

qemuarm64-secureboot is using systemd for uefi-secureboot, which has
warnings with musl (and fails to compile with clang and musl).  So,
modify the matrix to keep the coverage of everything else but musl.

Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -263,6 +263,9 @@ qemuarm64-secureboot:
       - KERNEL: [linux-yocto, linux-yocto-rt]
         TOOLCHAINS: [gcc, clang]
         TCLIBC: [glibc, musl]
+        TS: [none, qemuarm64-secureboot-ts]
+        TESTING: testimage
+      - TOOLCHAINS: [gcc, clang]
         TS: [none, qemuarm64-secureboot-ts]
         UEFISB: [none, uefi-secureboot]
         TESTING: testimage