chore(xtask): refactor tool runners & e2e test supergraph demo

[EverlastingBugstopper]

The first commit is a refactor that cleans up a bit of rushed xtask code.

The second/third commit is the following:

1. Iff running on GNU linux
2. clone https://github.com/apollographql/supergraph-demo into a temp directory
3. run make ci
4. assert that the output includes the following output:

"🚀 Graph Router ready at http://localhost:4000/", "allProducts",  and "Removing network"

There is an argument to be made here that supergraph-demo should live as a subdirectory in rover if we want it to be part of our test suite, but figured that this was an easier path forward and lets us keep git history intact w/o resorting to the hellscape that is git submodules.

These tests will only run in cargo xtask test commands and not cargo test commands, so they should not be too much of a burden for the average contributor (they don't need to install make or docker or anything if they just run cargo test).