Added list of env variables for fn-plugins

KRM-functions should be able get env variables.
Reflecting in `build` cmd args added in [1]
for `fn run` cmd.

[1]
https://github.com/kubernetes-sigs/kustomize/pull/2988/

kustomize/internal/commands/build/build.go

@@ -90,6 +90,9 @@ func NewCmdBuild(out io.Writer) *cobra.Command {
 	cmd.Flags().StringArrayVar(
 		&o.fnOptions.Mounts, "mount", []string{},
 		"a list of storage options read from the filesystem")
+	cmd.Flags().StringArrayVarP(
+		&o.fnOptions.Env, "env", "e", []string{},
+		"a list of environment variables to be used by functions")
 
 	addFlagLoadRestrictor(cmd.Flags())
 	addFlagEnablePlugins(cmd.Flags())