[FR] Expand `CUSTOM_RULES_DIR` to support user-relative paths

[pberba]

Repository Feature

None

Problem Description

Currently the CUSTOM_RULES_DIR environment variable only accepts an absolute paths.

Desired Solution

Modify https://github.com/elastic/detection-rules/blob/5d69eb19badf1af854b3e5cdb9445f214b733ead/detection_rules/config.py#L230

to

path = Path(CUSTOM_RULES_DIR).expanduser() / "_config.yaml"

Considered Alternatives

No response

Additional Context

No response

[botelastic[bot]]

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