Change Runner src in vagrant configuraiton for Windows development

What does this MR do?

Change Runner src on Windows development

Why was this MR needed?

We are using go modules and they don't behave when inside of the GOROOT:

PS C:\Go\src\gitlab.com\gitlab-org\gitlab-runner> go test -v gitlab.com/gitlab-org/gitlab-runner/helpers/process
go: inconsistent vendoring in C:\Go\src\gitlab.com\gitlab-org\gitlab-runner:
        go.mod requires gitlab.com/gitlab-org/gitlab-runner  but vendor/modules.txt does not include it.
        run 'go mod tidy; go mod vendor' to sync

The Runner source code doesn't have to be inside of the GOROOT/GOPATH anymore since it's using modules.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased