Add missing po4a requirement for Ubuntu 20.04 on WSL 2 in bootstrap.sh

1 job for updated_bootstrap in 5 minutes and 39 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
failed #29834
img

00:05:39

 
Name Stage Failure
failed
img Test
mv "target/x86_64-unknown-redox"/openlibm.partial "target/x86_64-unknown-redox"/openlibm
touch "target/x86_64-unknown-redox"/openlibm
make[2]: Leaving directory '/builds/IceTDrinker/redox/relibc'
make[1]: *** [prefix/x86_64-unknown-redox/relibc-install] Error 2
make: *** [ci-img] Error 2
mk/prefix.mk:19: recipe for target 'prefix/x86_64-unknown-redox/relibc-install' failed
make[1]: Leaving directory '/builds/IceTDrinker/redox'
Makefile:69: recipe for target 'ci-img' failed
ERROR: Job failed: exit code 1