Don't scrub the environment when Firefox executes itself (refs: #15704).

It causes this error:

  ERROR: ld.so: object 'libmozsandbox.so' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.

… presumably because LD_LIBRARY_PATH is not set due to the "P" profile
transition flag.

config/chroot_local-includes/usr/share/tails/torbrowser-AppArmor-profile.patch

@@ -123,7 +123,7 @@ index d0aded9..3be3872 100644
 +  deny /usr/local/lib/tor-browser/TorBrowser/UpdateInfo/ rw,
 +  deny /usr/local/lib/tor-browser/update.test/ rw,
 +
-+  /usr/local/lib/tor-browser/firefox Pix,
++  @{torbrowser_firefox_executable} px -> torbrowser_firefox,
 +
 +  # Grant access to assistive technologies
 +  # (otherwise, Firefox crashes when Orca is enabled: