pin `vt-tv` version in build script

[cz4rs]

Create a new vt-tv release and use that in build_cpp.sh:

vt/ci/build_cpp.sh

Lines 111 to 112 in a396d17

	vt_tv_rev="master"
	git clone -b "${vt_tv_rev}" --depth 1 https://github.com/DARMA-tasking/vt-tv.git

Note: this is more geared towards vt releases, could be good to stick with the latest version when we run the CI on regular PRs 🤷