fix(scripts): use TMPDIR-aware mktemp template in shell scripts

[mesutoezdil]

Replaces bare mktemp calls with mktemp "${TMPDIR:-/tmp}/openshell-*.XXXXXX" in 2 scripts

Consistent with the pattern already used on line 227 of helm-k3s-local.sh

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[mesutoezdil]

Closing -- this is a consistency improvement rather than a real bug, and there are already several PRs open for review.

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