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/openvpn/extract/pem.go

@@ -4,15 +4,12 @@ import (
 	"encoding/base64"
 	"encoding/pem"
 	"errors"
-	"fmt"
 )
 
-var errPEMDecode = errors.New("cannot decode PEM encoded block")
-
 func PEM(b []byte) (encodedData string, err error) {
 	pemBlock, _ := pem.Decode(b)
 	if pemBlock == nil {
-		return "", fmt.Errorf("%w", errPEMDecode)
+		return "", errors.New("cannot decode PEM encoded block")
 	}
 
 	der := pemBlock.Bytes