Specifying `command-subdir` doesn't change the element's cache key

Summary

The command-subdir variable from any BuildElement is not taken into account in the cache key's calculation, making it possible to have two elements share the same key.

Steps to reproduce

git clone https://gitlab.com/BenjaminSchubert/bst-bugs
cd bst-bugs/
bst show

Both elem1 and elem2 will have the same cache key. Although their command-subdir is different.

Those are dummy elements just for demonstration purposes, but real life scenarios might require multiple sources checked out and then build from one of them. It is therefore possible to get conflicts in real projects.