F-36: rebuild against ruby31

modernize spec file, especially move %gem_install to %build
  to fix FTBFS with package_notes

rubygem-redcarpet.spec

@@ -27,14 +27,15 @@ BuildArch: noarch
 Documentation for %{name}.
 
 %prep
-%setup -q -c -T
-%gem_install -n %{SOURCE0}
+%setup -q -n %{gem_name}-%{version}
+
+%build
+gem build ../%{gem_name}-%{version}.gemspec
+%gem_install
 
 # https://github.com/vmg/redcarpet/pull/503
 chmod a-x .%{gem_instdir}/ext/redcarpet/html.c
 
-%build
-
 %install
 mkdir -p %{buildroot}%{gem_dir}
 cp -a ./%{gem_dir}/* %{buildroot}%{gem_dir}/
@@ -76,6 +77,8 @@ popd
 %changelog
 * Wed Jan 26 2022 Mamoru TASAKA <mtasaka@fedoraproject.org> - 3.3.2-22
 - F-36: rebuild against ruby31
+- modernize spec file, especially move %%gem_install to %%build
+  to fix FTBFS with package_notes
 
 * Fri Jan 21 2022 Fedora Release Engineering <releng@fedoraproject.org> - 3.3.2-21
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_36_Mass_Rebuild