mkosi: Override rpm's _fixperms script to be a noop

Since rpm 4.20 it tries to fix the permissions of all source files
in the sources. This takes forever and fails in our case with a
permission error. Since we don't want rpm touching our source files,
override the script to be a noop.

mkosi.images/system/mkosi.conf.d/10-centos-fedora/mkosi.build.chroot

@@ -77,6 +77,7 @@ ANNOBIN="no-active-checks" rpmbuild \
     --define "__script_requires %{nil}" \
     --define "_find_debuginfo_dwz_opts %{nil}" \
     --define "_fortify_level 0" \
+    --define "_fixperms true" \
     --undefine _lto_cflags \
     --undefine _package_note_flags \
     --noclean \

mkosi.images/system/mkosi.conf.d/10-opensuse/mkosi.build.chroot

@@ -76,6 +76,7 @@ build() {
         --define "__elf_exclude_path ^/usr/lib/systemd/tests/unit-tests/.*$" \
         --define "__script_requires %{nil}" \
         --define "_find_debuginfo_dwz_opts %{nil}" \
+        --define "_fixperms true" \
         --noclean \
         "$@" \
         "pkg/$ID/systemd.spec"