nixos-enter: inherit parent PID namespace

Removes the creation of a new PID namespace during nixos-enter so that
systemd doesn't detect the child process as being in a "container". In
systemd v257, container detection slightly changed to check if the
process is a part of the root PID namespace (see https://github.com/systemd/systemd/blob/96c4d9d94d06c6c0a8b68be376505f8d8b5eba2b/src/basic/virt.c#L735).
Tooling such as `bootctl` will not perform certain actions if it detects
we are in a container, such as not populating EFI variables. This
results in broken systemd-boot VM tests.

pkgs/by-name/ni/nixos-enter/nixos-enter.sh

@@ -12,7 +12,7 @@ if [ -z "$NIXOS_ENTER_REEXEC" ]; then
     if [ "$(id -u)" != 0 ]; then
         extraFlags="-r"
     fi
-    exec unshare --fork --mount --uts --mount-proc --pid $extraFlags -- "$0" "$@"
+    exec unshare --fork --mount --uts --mount-proc $extraFlags -- "$0" "$@"
 else
     mount --make-rprivate /
 fi