Set the reported plugin version at release build time #35

The v0.11.0 release assets are named and indexed as v0.11.0, but crane transform list-plugins reports v0.1.0 because openshift.PluginVersion is a hard-coded constant. This makes the installed binary disagree with the version selected from the plugin index.

Adopt the pattern used by crane-plugin-buildconfig-to-builds:

• Change PluginVersion to an exported package variable initialized to "devel".
• Build release binaries with -ldflags "-X github.com/migtools/crane-plugin-openshift/openshift.PluginVersion=${VERSION}".
• Add a release-build check that each generated binary contains ${VERSION}, so a stale -X package path fails the release.
• Do not register a version in crane-plugins until the corresponding artifacts have passed that check.

The release workflow should also avoid reusing release tags, so an indexed version always refers to immutable artifacts.

This issue covers the release workflow and a corrected release after the implementation is merged.