MoltenVK: fix livecheck to exclude release candidates

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

graphics/MoltenVK/Portfile

@@ -29,9 +29,11 @@ variant universal   {}
 # it here, the GitHub website will get saved as the value for
 # ${homepage} for all of the subports' 'port info' entries.
 github.setup        KhronosGroup ${name} ${version} v
+github.livecheck.regex  {([0-9]+\.[0-9]+\.[0-9]+)}
 
 if {${subport} eq ${name}} {
     PortGroup       stub 1.0
+
     revision        0
     epoch           2
     supported_archs noarch