[CI] Enable gnuinstall for two special builds.

To cover the gnuninstall case, switch mac beta and the linux arch-native
builds to gnuinstall.
This is done as an option override on special builds to not affect the
binaries we produce.

Author: hageboeck

.github/workflows/root-ci.yml

@@ -112,7 +112,7 @@ jobs:
           - platform: mac-beta
             is_special: true
             arch: ARM64
-            overrides: ["CMAKE_CXX_STANDARD=23"]
+            overrides: ["CMAKE_CXX_STANDARD=23", "gnuinstall=ON"]
 
     runs-on: # Using '[self-hosted, ..., ...]' does not work for some reason :)
       - self-hosted
@@ -400,8 +400,8 @@ jobs:
             overrides: ["runtime_cxxmodules=Off"]
           - image: alma9
             is_special: true
-            property: march_native
-            overrides: ["CMAKE_BUILD_TYPE=RelWithDebInfo", "CMAKE_CXX_FLAGS=-march=native", "CMAKE_C_FLAGS=-march=native", "builtin_zlib=ON", "builtin_zstd=ON"]
+            property: "march_native, gnuinstall"
+            overrides: ["CMAKE_BUILD_TYPE=RelWithDebInfo", "CMAKE_CXX_FLAGS=-march=native", "CMAKE_C_FLAGS=-march=native", "builtin_zlib=ON", "builtin_zstd=ON", "gnuinstall=ON"]
           - image: alma9
             is_special: true
             property: arm64