karmadactl init: use karmadaBootstrappingLabelKey constant

[carlory]

What type of PR is this? /kind cleanup

Does this PR introduce a user-facing change?:

NONE

[karmada-bot]

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[carlory]

/cc @lonelyCZ @prodanlabs

[carlory]

/assign @prodanlabs

[carlory]

Please take a look @lonelyCZ @prodanlabs