[Fix] nvm run: avoid an unbound NVM_IOJS under set -u

[jayhemnani9910]

nvm run <version> only sets NVM_IOJS for io.js versions, then reads it, so in a script with set -u it aborts for any node version:

set -u
. "$NVM_DIR/nvm.sh"
nvm run 16 -e 'console.log(1)'
# nvm.sh: line 4970: NVM_IOJS: unbound variable

This picks up #3698 by @Deco: it sets the variable to false, in the two-line form requested there for ksh. Added a fast test that fails without the fix in bash and dash.

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