Improve toolchain handling

Configure environment to avoid toolchain installs

Force go to always use the local toolchain (i.e. the one the one that shipped with the go command being run) via setting the GOTOOLCHAIN environment variable to local[1]:

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

This is how things are setup in the official Docker images (e.g.[2], see also the discussion around that change[3]). The motivation behind this is to:
- Reduce duplicate work, the action will install a version of Go, a toolchain will be detected, the toolchain will be detected and then another version of Go installed
- Avoid Unexpected behaviour: if you specify this action runs with some Go version (e.g. 1.21.0) but your go.mod contains a toolchain or go directive for a newer version (e.g. 1.22.0) then, without any other configuration/environment setup, any go commands will be run using go 1.22.0
This will be a breaking change for some workflows. Given a go.mod like:
```
module proj

go 1.22.0
```
Then running any go command, e.g. go mod tidy, in an environment where only go versions before 1.22.0 were installed would previously trigger a toolchain download of Go 1.22.0 and that version being used to execute the command. With this change the above would error out with something like:

go: go.mod requires go >= 1.22.0 (running go 1.21.7; GOTOOLCHAIN=local)

[1] https://go.dev/doc/toolchain#select [2] https://github.com/docker-library/golang/blob/dae3405a325073e8ad7c8c378ebdf2540d8565c4/Dockerfile-linux.template#L163 [3] https://github.com/docker-library/golang/issues/472
Prefer installing version from toolchain directive

Prefer this over the version from the go directive. Per the docs[1]

The toolchain line declares a suggested toolchain to use with the module or workspace

It seems reasonable to use this, since running this action in a directory containing a go.mod (or go.work) suggests the user is wishing to work with the module or workspace.

Check list:

[ ] Mark if documentation changes are required.
[ ] Mark if tests were added or updated to cover the changes.

actions / setup-go

Improve toolchain handling #460