[2.5] Fix flower app path

[yanchengnv]

Fixes # .

Description

Since the supernode is started with its current working directory set to the client app dir, if the custom directory is not an absolute path, it needs to be passed relative to the app dir.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[chesterxgchen]

LGTM

[nvidianz]

/build