Re-add PODMAN_USERNS environment variable

This was lost during the Podman 2.0 migration. Turns out to be a
very easy fix, fortunately - we want to use the environment var
if not explicitly overridden.

Fixes containers#6705

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

cmd/podman/common/create.go

@@ -464,7 +464,7 @@ func GetCreateFlags(cf *ContainerCLIOpts) *pflag.FlagSet {
 		"Username or UID (format: <name|uid>[:<group|gid>])",
 	)
 	createFlags.String(
-		"userns", "",
+		"userns", os.Getenv("PODMAN_USERNS"),
 		"User namespace to use",
 	)
 	createFlags.String(