[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