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/tun/check.go

@@ -9,11 +9,6 @@ import (
 	"syscall"
 )
 
-var (
-	ErrTUNInfo    = errors.New("cannot get syscall stat info of TUN file")
-	ErrTUNBadRdev = errors.New("TUN file has an unexpected rdev")
-)
-
 // Check checks the tunnel device specified by path is present and accessible.
 func (t *Tun) Check(path string) error {
 	f, err := os.OpenFile(path, os.O_RDWR, 0)
@@ -29,13 +24,13 @@ func (t *Tun) Check(path string) error {
 
 	sys, ok := info.Sys().(*syscall.Stat_t)
 	if !ok {
-		return fmt.Errorf("%w", ErrTUNInfo)
+		return errors.New("cannot get syscall stat info of TUN file")
 	}
 
 	const expectedRdev = 2760 // corresponds to major 10 and minor 200
 	if sys.Rdev != expectedRdev {
-		return fmt.Errorf("%w: %d instead of expected %d",
-			ErrTUNBadRdev, sys.Rdev, expectedRdev)
+		return fmt.Errorf("TUN file has an unexpected rdev: %d instead of expected %d",
+			sys.Rdev, expectedRdev)
 	}
 
 	if err := f.Close(); err != nil {