integration: remove use of golang.org/x/sys/execabs

This dependency was removed in 2af6db672e, but
was re-introduced in commit 2fab240f21.

Now that golang.org/x/tools also stopped using this dependency, removing
this use will remove the package from our dependency tree.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/release_upgrade_utils_linux_test.go

@@ -21,13 +21,13 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"os/exec"
 	"runtime"
 	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 	"golang.org/x/mod/semver"
-	exec "golang.org/x/sys/execabs"
 
 	"github.com/containerd/containerd/v2/archive"
 	"github.com/containerd/containerd/v2/version"