Move file manipulation to %build

Without this, in-place builds are broken: they don't execute %prep, so
%install fails.

[skip changelog]

systemd.spec

@@ -556,8 +556,6 @@ package and is meant for use in exitrds.
 %prep
 %autosetup -n %{?commit:%{name}%[%stable?"-stable":""]-%{commit}}%{!?commit:%{name}%[%stable?"-stable":""]-%{version_no_tilde}} -p1
 
-sed -r 's|/system/|/user/|g' %{SOURCE16} >10-timeout-abort.conf.user
-
 %generate_buildrequires
 %if 0%{?have_gnu_efi}
 if grep -q gnu-efi meson_options.txt; then
@@ -711,6 +709,8 @@ if ! diff -u %{SOURCE1} ${new_triggers}; then
    sleep 5
 fi
 
+sed -r 's|/system/|/user/|g' %{SOURCE16} >10-timeout-abort.conf.user
+
 %install
 %meson_install