Fix Typo and Improve Variable Default Value Setting

Valery Ushakov wrote...

This is a minor (and so far inconsequential) typo, so I don't want to create a fully blown PR just for that.

In https://gitlab.alpinelinux.org/alpine/cloud/tiny-cloud/-/blob/main/sbin/imds-net-sync#L14 there's a typo in

  PHASE=pre=down

with the equal sign, which should be pre-down with the minus sign (that is tested at the end but there's nothing in that case so far).

PS: Also I notice that tiny-cloud scripts consistently use

  VAR=${VAR:-default}

the common idiom for this is usually

  : ${VAR:=default}

which avoids spelling the variable name twice, reducing the cognitive load on the reader and the chance of a typo in the repeated name.