hooks: set expected environment when executing

During normal plugin execution (from the CLI), the CLI configures the
plugin command it's about to execute in order to pass all environment
variables on, as well as to set the ReExec env var that informs the
plugin about how it was executed, and which plugins rely on to check
whether they are being run standalone or not.

This commit adds the same behavior to hook invocations, which is
necessary for some plugins to know that they are not running standalone
so that they expose their root command at the correct level.

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

cli-plugins/manager/plugin.go

@@ -2,6 +2,7 @@ package manager
 
 import (
 	"encoding/json"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"regexp"
@@ -113,7 +114,10 @@ func (p *Plugin) RunHook(cmdName string, flags map[string]string) ([]byte, error
 		return nil, wrapAsPluginError(err, "failed to marshall hook data")
 	}
 
-	hookCmdOutput, err := exec.Command(p.Path, p.Name, HookSubcommandName, string(hDataBytes)).Output()
+	pCmd := exec.Command(p.Path, p.Name, HookSubcommandName, string(hDataBytes))
+	pCmd.Env = os.Environ()
+	pCmd.Env = append(pCmd.Env, ReexecEnvvar+"="+os.Args[0])
+	hookCmdOutput, err := pCmd.Output()
 	if err != nil {
 		return nil, wrapAsPluginError(err, "failed to execute plugin hook subcommand")
 	}