Actually disable credential cache for krb5 provisioner

[nightkr]

Description

KRB5CCNAME=MEMORY will store the cache in a file named MEMORY, KRB5CCNAME=MEMORY: will store the cache in memory (as intended).

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[nightkr]

Haven't tested it here, but ran into the same issue on stackabletech/opa-operator#524 and the same change fixed it there.

[sbernauer]

Nice catch