Standardize usage of `DATADOG_DIR`

[sarah-witt]

What does this PR do?

Standardizes the usage of DATADOG_DIR in the buildpack scripts.

Description of the Change

Some things the PR does are:

• Properly handles the variable (and some other variables) in the case of word-splitting
• Removes the path prefix in cases where we push the directory, ie when starting dogstatsd ./dogstatsd start --cfgpath dist/ > /dev/null

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.