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/routing/ip.go

@@ -1,7 +1,6 @@
 package routing
 
 import (
-	"errors"
 	"fmt"
 	"net"
 	"net/netip"
@@ -14,8 +13,6 @@ func ipIsPrivate(ip netip.Addr) bool {
 		ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast()
 }
 
-var errInterfaceIPNotFound = errors.New("IP address not found for interface")
-
 func ipMatchesFamily(ip netip.Addr, family uint8) bool {
 	return (family == netlink.FamilyV4 && ip.Is4()) ||
 		(family == netlink.FamilyV6 && ip.Is6())
@@ -46,6 +43,6 @@ func (r *Routing) AssignedIP(interfaceName string, family uint8) (ip netip.Addr,
 
 		return ip, nil
 	}
-	return ip, fmt.Errorf("%w: interface %s in %d addresses",
-		errInterfaceIPNotFound, interfaceName, len(addresses))
+	return ip, fmt.Errorf("IP address not found for interface: interface %s in %d addresses",
+		interfaceName, len(addresses))
 }