package/runc: bump to version 1.0.0-rc6

package/runc: bump to version 1.0.0-rc6

Previously, a specific commit hash from the Docker runc.installer was
used to determine the required runc version for the Docker
Engine. This old commit hash used was a untagged pre-1.0.0 release of
runc, closer to an earlier release candidate.

The runc version used in the Debian distribution is not the pinned
version previously used by Buildroot. It is the latest release
candidate. The latest release candidate is known to be compatible with
the Docker Engine, and there is no justification for pinning to an
older RC anymore.

This commit bumps to the latest RC, 1.0.0-rc6. A v1.0.0 is expected
soon.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>