Respect $XDG_DATA_HOME when looking for binaries

Since we're using the XDG base dir package in yabridgectl we were
already doing this there, so it makes sense to also do this in yabridge
itself even though it's very unlikely the user will have this set.

CHANGELOG.md

@@ -34,6 +34,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   allocations in certain situations. This is related to the similar issue that
   got fixed with yabridge 3.3.0. A fix for this issue has also been upstreamed
   to the library.
+- Respect `$XDG_DATA_HOME` when looking for yabridge's plugin host binaries
+  instead of hardcoding `~/.local/share/yabridge`. This matches the existing
+  behaviour in yabridgectl.
 
 ### Fixed