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.

compiler/compiler_test.go

@@ -29,7 +29,7 @@ func TestCompiler(t *testing.T) {
 	t.Parallel()
 
 	// Determine Go minor version (e.g. 16 in go1.16.3).
-	_, goMinor, err := goenv.GetGorootVersion(goenv.Get("GOROOT"))
+	_, goMinor, err := goenv.GetGorootVersion()
 	if err != nil {
 		t.Fatal("could not read Go version:", err)
 	}