Make go-version-file: go.mod the default

[andig]

Description:
Currently, I have to add

with:
  go-version-file: go.mod

for all builds to specify a go version. Instead, the "default" go version that any go program would use is the one from the go.mod with fallback to the installed go version. This should be the default for setup-go, too.

Justification:
Simplify developer's life.

Are you willing to submit a PR?

[mahalakshmi-rekadi]

Hi, @andig 👋 Thank you for the feature request, we will investigate it and get back to you as soon as we have some feedback.

[Jacalz]

This seems like a helpful change. If not Go version is specified we load the one from the module and anyone that wants to test oldest supported and newest, we can do something like #450.

[andig]

Thinking about it I'm no longer too happy with this request. If go.mod specifies the minimum version- and the go command will upgrade to that toolchain if not already installed- then the version for compiling should be the current stable version. It is just not entirely clear to me, what the default version of setup-go even is if no version is specified?