action: do not expand env vars in Exec action

action: do not expand env vars in Exec action

...<command> argument (but still resolve tilde).

This makes it easier to write sh -c '' constructs without turning labwc
into a shell parser in order to expand environment variables, whilst
respecting single quotes and escaped characters as well as ignoring
subshells syntax like $(foo) and backticks.

Also, fix bug where buffer length+alloc get out-of-sync