Bump git-sync version to v4.4.0

[ChenYi015]

Purpose of this PR

Proposed changes:

• Bump git-sync to v4.4.0 and update git-sync image
• Replace deprecated GIT_SYNC_* environment variables with the GITSYNC_* ones, including:
  • GIT_SYNC_REPO -> GITSYNC_REPO
  • GIT_SYNC_ROOT -> GITSYNC_ROOT
  • GIT_SYNC_ONE_TIME -> GITSYNC_ONETIME

Note:

• Env GIT_SYNC_DEST is kept the same as before as it was not renamed to GITSYNC_DEST in git-sync.

Change Category

• Bugfix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that could affect existing functionality)
• Documentation update

Rationale

git-sync ref: https://github.com/kubernetes/git-sync/blob/69eb59185a073d4a08362d07bbe6459311027746/main.go.

[google-oss-prow]

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