command: list releases only for remote installs

When installing a plugin from a remote source, we list the installed
plugins that match the constraints specified, and if the constraint is
already satisfied, we don't do anything.

However, since remote installation is only relevant for releases of a
plugin, we should only look at the installed releases of a plugin, and
not consider pre-releases for that step.

This wasn't the case before this commit, as if a prerelease version of a
commit (ex: 10.8.1-dev), and we try to invoke `packer init` with a
constraint on this version specifically, Packer would locate that
pre-release and assume it was already installed, so would silently
succeed the command and do nothing.

This isn't the expected behaviour as we should install the final release
of that plugin, regardless of any prerelease installation of the plugin.
So this commit fixes that by only listing releases, so we don't report
the plugin being already installed if a prerelease is what's installed.

command/init.go

@@ -85,6 +85,7 @@ for more info.`)
 			Checksummers: []plugingetter.Checksummer{
 				{Type: "sha256", Hash: sha256.New()},
 			},
+			ReleasesOnly: true,
 		},
 	}
 

command/plugins_install.go

@@ -130,6 +130,7 @@ func (c *PluginsInstallCommand) RunContext(buildCtx context.Context, args *Plugi
 			Checksummers: []plugingetter.Checksummer{
 				{Type: "sha256", Hash: sha256.New()},
 			},
+			ReleasesOnly: true,
 		},
 	}
 	if runtime.GOOS == "windows" {