rls: prepend rustup path to PATH

Merged Nathaniel McCallum requested to merge npmccallum/gnome-builder:rlsfix into master

The existing code replaces PATH altogether. But this doesn't work when rustc ends up calling out to utilities like cc. The end result is that rls silently stops working. Instead, we will retain the existing path and merely add the rustup location to it. We put the rustup location first because this will most likely be the most up to date rust toolchain.

Fixes: #669 (closed)

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