chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/wireguard/config_test.go

@@ -38,7 +38,7 @@ func Test_makeDeviceConfig(t *testing.T) {
 			settings: Settings{
 				PrivateKey: "bad key",
 			},
-			err: ErrPrivateKeyInvalid,
+			err: errors.New("cannot parse private key"),
 		},
 		"bad public key": {
 			settings: Settings{