`BuildFlags.buildDirectoryFromWorkspacePath` hardcodes `.build` instead of using the `--scratch-path` if set

[plemarquand]

Describe the bug
The BuildFlags.buildDirectoryFromWorkspacePath doesn't take in to account the user's --scratch-path if they have it set via additional build args. This method should be updated and the code that uses it should be tested to make sure it takes in to account a custom build directory set by the user.

[kartikangiras]

will work on this one as well