Incomplete search path(s) for xdelta3 executable

When determining whether to use deltas or do a full snap download, snapd verifies that an xdelta3 executable is available. However it appears to only look for xdelta3 in (a) the system path, and (b) the core snap.

So in a situation where xdelta3 is not installed in the system path, and the snapd snap is used instead of the core snap, snapd's xdelta3 executable is not found. As a result, snapd does not use deltas but instead does full downloads.

The relevant code for 2.43.3 is at
https://github.com/snapcore/snapd/blob/2.43.3/store/store.go#L1678-L1686
under the getXdelta3Cmd function. Presumably a potential fix is to add filepath.Join(dirs.SnapMountDir, "/snapd/current/usr/bin/xdelta3") to the list of paths being searched.