Fix --config being ignored by loadConfig

Use `GlobalString` in `loadConfig` since `--config` is not a sub-command
flag.

This fixes `--config` being ignored and the CLI erroring out in a
maddening way.

FWIW here is how I got a debug build on Nix to debug this issue with
delve:

```
$ cat ~/snippets/sops-debug-build.nix
sops.overrideAttrs(final: prev: {
  ldflags = [ "-X github.com/getsops/sops/v3/version.Version=${prev.version}" ];
  GOFLAGS = prev.GOFLAGS ++ [ "'-gcflags=all=-N -l'" ];
  dontStrip = true;
})
$ nix-store --realise $(nix-instantiate ~/snippets/sops-debug-build.nix)
```

cmd/sops/main.go

@@ -2178,10 +2178,8 @@ func keyGroups(c *cli.Context, file string) ([]sops.KeyGroup, error) {
 // Since a config file is not required, this function does not error when one is not found, and instead returns a nil config pointer
 func loadConfig(c *cli.Context, file string, kmsEncryptionContext map[string]*string) (*config.Config, error) {
 	var err error
-	var configPath string
-	if c.String("config") != "" {
-		configPath = c.String("config")
-	} else {
+	configPath := c.GlobalString("config")
+	if configPath == "" {
 		// Ignore config not found errors returned from FindConfigFile since the config file is not mandatory
 		configPath, err = config.FindConfigFile(".")
 		if err != nil {