This replaces our own manual detection of various variables (GOROOT,
GOPATH, Go version) with a simple call to `go env`.
If the `go` command is not found:
error: could not find 'go' command: executable file not found in $PATH
If the Go version is too old:
error: requires go version 1.18 through 1.20, got go1.17
If the Go tool itself outputs an error (using GOROOT=foobar here):
go: cannot find GOROOT directory: foobar
This does break the case where `go` wasn't available in $PATH but we
would detect it anyway (via some hardcoded OS-dependent paths). I'm not
sure we want to fix that: I think it's better to tell users "make sure
`go version` prints the right value" than to do some automagic detection
of Go binary locations.
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