fix(builder): keep absolute paths

[rlodico]

Details

Relative paths do not work later on in Best when the rootDir and the
cwd are not the same because the relative paths are later re-resolved
against the cwd, which points to locations non-existent in the fs

Introduces assertions to existing build output test to validate that the output contains usable paths

Fixes #254

Does this PR introduce a breaking change?

• Yes
• No

[diervo]

Is this the only place where we have this issue?

[rlodico]

@diervo I didn't run into any more pathing issues besides this one.

Both a remote agent (with a headless runner) and a local headless runner worked and neither reported pathing issues after the change.

Edit: Looking again, there's actually only ever one usage of benchmarkFolder, and it's the usage that caused my crash in @best/store/src/index.ts#L41. This is where I was originally going to put the fix but if I recall correctly I would have needed to modify the methods to pass in more information than was already available from the result data in order to reconstruct the full path.