Copy of buildkit-qemu-emulator should ignore xattr failures

[rgov]

There are a number of bug reports in various places that look like the following.

executor failed running [/dev/.buildkit_qemu_emulator xyz]:
failed to copy xattrs: failed to set xattr "security.selinux" on
/tmp/buildkit-qemu-emulator538849571/dev/.buildkit_qemu_emulator:
operation not supported

The issue applies to systems with SELinux enabled; SELinux prevents the modification of an ACL attached to a file as an xattr.

The error arises when copying the emulator binary into a temporary location:

buildkit/solver/llbsolver/ops/exec_binfmt.go

Lines 67 to 72 in db6989a

	if err := copy.Copy(context.TODO(), filepath.Dir(m.path), filepath.Base(m.path), tmpdir, qemuMountName, func(ci *copy.CopyInfo) {
	m := 0555
	ci.Mode = &m
	}, copy.WithChown(uid, gid)); err != nil {
	return nil, nil, err
	}

The implementation of copy.Copy() is from tonistiigi/fsutil. The error string "failed to copy xattrs" originates here:

https://github.com/tonistiigi/fsutil/blob/0789dc562bd7099bec7be479164e261ac5334f5f/copy/copy.go#L425-L427

Several years ago the PR containerd/continuity#138 added an option to ignore xattr errors specifically due to this security.selinux issue. The tonistiigi/fsutil implementation mirrors this option, but it is unused in the BuildKit code.

I believe in the BuildKit code ci.XAttrErrorHandler should be set to a function that ignores ENOTSUP, as in this PR.

cc @tonistiigi.