Use RPM macros everywhere

rubberband.spec

@@ -57,8 +57,8 @@ developing applications that use %{name}.
 
 %install
 %meson_install
-find $RPM_BUILD_ROOT -name '*.la' -exec rm -f {} ';'
+find %{buildroot} -name '*.la' -exec rm -f {} ';'
-rm -rf $RPM_BUILD_ROOT%{_libdir}/*.a
+rm -rf %{buildroot}%{_libdir}/*.a
 
 
 %if %{with check}