main: use go env instead of doing all detection manually

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.

builder/config.go

@@ -1,7 +1,6 @@
 package builder
 
 import (
-	"errors"
 	"fmt"
 
 	"github.com/tinygo-org/tinygo/compileopts"
@@ -24,14 +23,9 @@ func NewConfig(options *compileopts.Options) (*compileopts.Config, error) {
 		spec.OpenOCDCommands = options.OpenOCDCommands
 	}
 
-	goroot := goenv.Get("GOROOT")
-	if goroot == "" {
-		return nil, errors.New("cannot locate $GOROOT, please set it manually")
-	}
-
-	major, minor, err := goenv.GetGorootVersion(goroot)
+	major, minor, err := goenv.GetGorootVersion()
 	if err != nil {
-		return nil, fmt.Errorf("could not read version from GOROOT (%v): %v", goroot, err)
+		return nil, err
 	}
 	if major != 1 || minor < 18 || minor > 20 {
 		// Note: when this gets updated, also update the Go compatibility matrix: