perl: Do not build in parallel
While 743f0c5b (perl: Run make depend to fix build race condition, 2024-03-05) fixed 98% of all parallel build failures there is still some flakiness left in the perl build process. For now, give up instead of digging deep into the perl build process to debug this.
We may later revert this commit to see whether the issue is fixed in a later perl version in a later OpenWrt release.
Signed-off-by: Andreas Gnau andreas.gnau@iopsys.eu