init: fix --force and pre-releases installed case

When packer init is invoked with a --force argument, but no --update, we
clamp the version to install based on the last one locally installed.

Doing this may however cause the constraint to always be false if the
latest available version of a plugin is a pre-release, as none of the
upstream constraints will match that.

Therefore this commit changes how the constraint is derived from the
local list of installations, so that only the last installation that
matches the original constraint will be used, and not a pre-release.

command/init.go

@@ -127,7 +127,20 @@ for more info.`)
 			}
 
 			if cla.Force && !cla.Upgrade {
-				pluginRequirement.VersionConstraints, _ = gversion.NewConstraint(fmt.Sprintf("=%s", installs[len(installs)-1].Version))
+				// Only place another constaint to the latest release
+				// binary, if any, otherwise this is essentially the same
+				// as an upgrade
+				var installVersion string
+				for _, install := range installs {
+					ver, _ := gversion.NewVersion(install.Version)
+					if ver.Prerelease() == "" {
+						installVersion = install.Version
+					}
+				}
+
+				if installVersion != "" {
+					pluginRequirement.VersionConstraints, _ = gversion.NewConstraint(fmt.Sprintf("=%s", installVersion))
+				}
 			}
 		}