fix(makefile): platform aware default, fixes start-local

[ranakan19]

#25780 introduces ARGOCD_E2E_DIR making it configurable for macOS for running e2e tests.

make target start-local uses the same procfile as e2e test and thus the same script for git-server - which defaults the value of this variable as tmp/argocd.

This PR uses a platform aware default for ARGOCD_E2E_DIR in the makeefile.

with this change, git-server shows up as running with goreman run status as expected.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
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• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
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[Mangaal]

LGTM

[ppapapetrou76]

LGTM 🏅 - thanks for addressing my comments