engine: use GOTOOLCHAIN=local

[bassosimone]

When using go >= 1.21.0, we would need to pin to the exact version of the toolchain that we're using for building.

See https://go.dev/doc/toolchain for documentation:

When GOTOOLCHAIN is set to local, the go command always runs the bundled Go toolchain.

This is why export GOTOOLCHAIN=local should be such to enforce the previous behavior.