Toolchains are created once and saved locally

[karkhaz]

Toolchains are now downloaded and built into a locally-mounted directory under the directory sysroots, rather than a data container. If a particular toolchain already exists in sysroots, then it will not be re-downloaded on subsequent invocations of tuscan build for that toolchain.

This is to facilitate reproducibility of results. Before this commit, there was no provision to load a pre-created toolchain into tuscan; each toolchain would be downloaded from the upstream every time tuscan was run. This commit will allow us to distribute pre-created toolchains to others; tuscan will then use those toolchains rather than downloading the latest version.