fix: use the correct toolchain in the math template

[arthurpaulino]

Some users might end up with incompatible Lean toolchains when using the math template.

This PR prevents that by cloning mathlib4 and then reading the correct toolchain from lake-packages/mathlib4/lean-toolchain instead of using the default toolchain.

[tydeu]

Thanks for the contribution! While this will get the right lean-toolchain version -- which is great -- it does not properly setup the mathlib package (i.e., write the package to the manifest). Thus, clones of the package will not have the right mathlib pinned and subsequent run of lake build will not properly include mathlib. I would advise taking a look at Load.Main and Load.Materialize and using/adapting the definitions their to build a proper manifest. Does that make sense?

[arthurpaulino]

it does properly setup the mathlib package

Did you mean it doesn't?

[tydeu]

Did you mean it doesn't?

Oops, yeah that was a typo. I fixed it.

[tydeu]

LGTM! Thanks!