use arch rather than hardcode x86_64 in espeak-ng-data paths

[mcarans]

Basically does what's in this PR:
#557

[mcarans]

@oocube @LW-archlinux are either of you able to test this?

[LW-archlinux]

install_packages_root.sh is only called from test_builds.yaml and build-all.yaml which are in .github/worklows folder .
It can only be tested on systems with access to github CI, probably limited to github.com/OoliteProject/oolite and forks within the github ecosystem.
Archlinux espeak-ng package uses /usr/share/espeak-ng-data/ and this is the upstream default path so the change should work for archlinux systems with espeak-ng installed and running on github CI.

As for paths surrounded by double quotes : paths without spaces in them don't need them.
If double-quoting all paths in oolite sourcecode is desired/deemed necessary that should be clarified in a coding style document.
Does oolite have such a document ?

I think post_build.sh is called by meson install so it should affect all builds ?
I'll have to apply this PR to master to test if that poses issues , will try to do that soon.