Cirrus CI: Adapt to ubuntu:rolling now being 26.04 (#5126)

Author: kinke

.cirrus.yml

@@ -12,8 +12,7 @@ clone_steps_template: &CLONE_STEPS_TEMPLATE
 common_steps_template: &COMMON_STEPS_TEMPLATE
   install_lit_script: |
     # Install lit
-    rm -f /usr/lib/python3.13/EXTERNALLY-MANAGED # required for Ubuntu 25.04
-    python3 -m pip install --user lit
+    python3 -m pip install --break-system-packages lit
     python3 -c "import lit.main; lit.main.main();" --version . | head -n 1
   clone_submodules_script: |
     cd $CIRRUS_WORKING_DIR
@@ -201,7 +200,7 @@ task:
   environment:
     CI_ARCH: x86_64
     CI_OS: linux
-    EXTRA_APT_PACKAGES: "gdmd llvm-dev libclang-common-20-dev lld libcurl4-openssl-dev"
+    EXTRA_APT_PACKAGES: "gdmd llvm-dev libclang-common-21-dev lld libcurl4-openssl-dev"
     # need `-DCMAKE_EXE_LINKER_FLAGS=-lcurl` to work around libcurl dependency for ldc-profdata (distro LLVM apparently built with non-default LLVM_ENABLE_CURL=ON)
     EXTRA_CMAKE_FLAGS: >-
       -DBUILD_SHARED_LIBS=ON