[cargo-nightly] Update version computing logic in Plan.
[cargo-nightly] Update version computing logic in Plan.

This updates the version computing logic to also recompute the newly
added `$SRC_PATH` and `$CACHE_PATH` build variables. As this method of
lazily computing the `pkg_version` isn't yet officially supported, I'm
not concerned with this fix. Future work will introduce a helper to make
this exact use case a first-class citizen. Until then, we fix!

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

be-plans/be,be-plans/be,be-plans/be,be-plans/be