Restore config loading, keep exported Version, keep cgo off

Bring back the in-progress viper and go-homedir configuration loading
from the original root command as a persistent pre-run on the root
command, with a --config flag and a config line in sutil info. Restore
the exported main.Version with a nolint directive instead of renaming
it. Export CGO_ENABLED=0 from the Makefile, drop -race from make test,
and remove the C toolchain from the Docker builder stage.

internal/cli/info.go

@@ -15,31 +15,41 @@ import (
 // module support.
 var ErrNoBuildInfo = errors.New("sutil: no build information in this binary")
 
+// noConfigFile is what `sutil info` prints when no config file was loaded.
+const noConfigFile = "(none)"
+
 // newInfoCommand returns `sutil info`, which prints the build version, the
-// Go toolchain version, the module path, and the VCS metadata recorded by
-// the Go toolchain at build time.
-func newInfoCommand(p Params) *cobra.Command {
+// Go toolchain version, the module path, the configuration file in use,
+// and the VCS metadata recorded by the Go toolchain at build time.
+func newInfoCommand(a *app) *cobra.Command {
 	return &cobra.Command{
 		Use:   "info",
 		Short: "Print build information",
 		Args:  cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, _ []string) error {
-			return writeBuildInfo(cmd.OutOrStdout(), p.Version)
+			return writeBuildInfo(
+				cmd.OutOrStdout(), a.params.Version, a.configFileUsed(),
+			)
 		},
 	}
 }
 
 // writeBuildInfo writes one "key: value" line per fact to w.
-func writeBuildInfo(w io.Writer, version string) error {
+func writeBuildInfo(w io.Writer, version, configFile string) error {
 	info, ok := debug.ReadBuildInfo()
 	if !ok {
 		return ErrNoBuildInfo
 	}
 
+	if configFile == "" {
+		configFile = noConfigFile
+	}
+
 	lines := []string{
 		"version: " + version,
 		"go: " + info.GoVersion,
 		"module: " + info.Main.Path,
+		"config: " + configFile,
 	}
 
 	for _, setting := range info.Settings {