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/cli.go

@@ -7,6 +7,7 @@ package cli
 
 import (
 	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
 )
 
 // Params configures New.
@@ -14,23 +15,48 @@ type Params struct {
 	// Version is the version string embedded at build time. It is shown
 	// by `sutil version`, `sutil --version`, and `sutil info`.
 	Version string
+
+	// ConfigDir is the directory searched for the optional .sneak
+	// configuration file. Empty means the user's home directory; tests set
+	// it to a temporary directory so the real home is never consulted.
+	ConfigDir string
+}
+
+// app carries the state shared by every command: the build parameters,
+// the --config flag, and the configuration loaded before any command runs.
+type app struct {
+	params     Params
+	configFile string
+	config     *viper.Viper
 }
 
 // New builds the root sutil command with every subcommand attached.
 func New(p Params) *cobra.Command {
+	a := &app{params: p, configFile: "", config: nil}
+
 	root := &cobra.Command{
 		Use:   "sutil",
 		Short: "sneak's shell utility",
 		Long: `sutil is a small collection of shell utilities built on the packages in
 sneak.berlin/go/x: fast pseudorandom byte streams, memorable passwords,
-BIP39 mnemonics, and crypto_box keypairs derived from them.`,
-		Version:      p.Version,
-		SilenceUsage: true,
+BIP39 mnemonics, and crypto_box keypairs derived from them.
+
+Before any command runs, sutil loads the optional configuration file named
+by --config, or else ~/.sneak.yaml (or .json, .toml, ...) if it exists, and
+makes environment variables available alongside it.`,
+		Version:           p.Version,
+		SilenceUsage:      true,
+		PersistentPreRunE: a.loadConfig,
 	}
 
+	root.PersistentFlags().StringVar(
+		&a.configFile, "config", "",
+		"config file (default is "+configName+".* in the home directory)",
+	)
+
 	root.AddCommand(
 		newVersionCommand(p),
-		newInfoCommand(p),
+		newInfoCommand(a),
 		newGenerateCommand(),
 		newCBKCommand(),
 	)