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/config.go

@@ -0,0 +1,73 @@
+package cli
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/mitchellh/go-homedir"
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+)
+
+// configName is the base name of the optional configuration file looked
+// up in the config directory; viper tries every extension it supports, so
+// ~/.sneak.yaml, ~/.sneak.json, ~/.sneak.toml, and so on all work.
+const configName = ".sneak"
+
+// loadConfig runs before every command. It reads the optional
+// configuration file into a.config and makes environment variables
+// available through it. The file is the one named by --config, or else
+// configName in Params.ConfigDir, which defaults to the user's home
+// directory. A missing default file is not an error; a file that is named
+// explicitly but missing, or one that cannot be parsed, is.
+func (a *app) loadConfig(cmd *cobra.Command, _ []string) error {
+	configDir := a.params.ConfigDir
+	if configDir == "" {
+		home, err := homedir.Dir()
+		if err != nil {
+			return fmt.Errorf("finding home directory: %w", err)
+		}
+
+		configDir = home
+	}
+
+	config := viper.New()
+	config.AddConfigPath(configDir)
+	config.SetConfigName(configName)
+	config.AutomaticEnv()
+
+	if a.configFile != "" {
+		config.SetConfigFile(a.configFile)
+	}
+
+	err := config.ReadInConfig()
+
+	var notFound viper.ConfigFileNotFoundError
+	if errors.As(err, &notFound) {
+		a.config = config
+
+		return nil
+	}
+
+	if err != nil {
+		return fmt.Errorf("reading config file: %w", err)
+	}
+
+	a.config = config
+
+	_, err = fmt.Fprintln(
+		cmd.ErrOrStderr(), "Using config file:", config.ConfigFileUsed(),
+	)
+
+	return err
+}
+
+// configFileUsed returns the path of the configuration file that
+// loadConfig read, or an empty string if there was none.
+func (a *app) configFileUsed() string {
+	if a.config == nil {
+		return ""
+	}
+
+	return a.config.ConfigFileUsed()
+}