Cirrus-CI: split main script into separate world + kernel

It appears that Cirrus-CI has a 100MB limit for log output, and we
exceed that (!) with the amd64-gcc12 build.  Separate world and kernel
build tasks in an attempt to stay below the limit.

This also has the benefit of showing world and kernel build status
separately in the Cirrus-CI UI.

PR:		271903
Sponsored by:	The FreeBSD Foundation

.cirrus.yml

@@ -73,8 +73,11 @@ task:
   - mkdir -p /usr/obj/$(pwd -P)
   - chown user:user /usr/obj/$(pwd -P)
 
-  script:
-  - su user -c "make -j$(sysctl -n hw.ncpu) CROSS_TOOLCHAIN=${TOOLCHAIN} WITHOUT_TOOLCHAIN=yes buildworld buildkernel"
+  build_world_script:
+  - su user -c "make -j$(sysctl -n hw.ncpu) CROSS_TOOLCHAIN=${TOOLCHAIN} WITHOUT_TOOLCHAIN=yes buildworld"
+
+  build_kernel_script:
+  - su user -c "make -j$(sysctl -n hw.ncpu) CROSS_TOOLCHAIN=${TOOLCHAIN} WITHOUT_TOOLCHAIN=yes buildkernel"
 
   package_script:
   - su user -c "make CROSS_TOOLCHAIN=${TOOLCHAIN} WITHOUT_TOOLCHAIN=yes packages"