[ci] Enable gnuinstall in the march=native build

This is to test that ROOT doesn't get confused at build and test time by
the fact that the structure of the install tree and build tree is
different, which is the case for `gnuinstall=ON`.

Author: guitargeek

.github/workflows/root-ci-config/buildconfig/alma9-march_native.txt

@@ -6,6 +6,7 @@ builtin_vdt=ON
 builtin_veccore=ON
 builtin_zlib=ON
 builtin_zstd=ON
+gnuinstall=ON
 pythia8=ON
 tmva-sofie=ON
 veccore=ON

.github/workflows/root-ci.yml

@@ -128,6 +128,11 @@ jobs:
           - platform: mac-beta
             is_special: true
             arch: ARM64
+            # For the PR and incremental builds, we set gnuinstall=ON to check
+            # if CMake can deal with the different structure of the install and
+            # build tree at build and test time. We don't care how the install
+            # actually looks like.
+            overrides: ["gnuinstall=ON"]
 
     runs-on:
       - self-hosted