Moving and exporting this variable from the main to the goenv package
allows us to use it from both the main and the builder package.
This is done in preparation to include the value in `tinygo build`
linker flags, so that we can embed the version and git sha into binaries
built with tinygo.
First look at the VERSION file, only then look at
src/runtime/internal/sys/zversion.go. This makes it possible to
correctly detect the Go version for release candidates.
This is needed to make it available to more packages, for caching
purposes.
For caching, the version itself may not be enough during development.
But for regular releases, the version provides some protection against
accidentally using a cache entry that is invalid in a newer version.
This is necessary to avoid a circular dependency in the loader (which
soon will need to read the Go version) and because it seems like a
better place anyway.
