vmimage.subr: pkg autoremove after pkg install

A bug in pkg, which somehow only surfaced as a consequence of pkgbase,
results in pkg install sometimes pulling in false dependencies. This
problem might be limited to cases when the lib32 pkgbase packages are
not installed. In the case of EC2 "small" images, installing the
ebsnvme-id package results in binutils, gcc12-devel, gmp, indexinfo,
liblz4, mpc, mpfr, and zstd packages being installed.

These false dependencies are however not recorded as dependencies --
at some level pkg does understand that they're not needed -- so running
pkg autoremove immediately after pkg install cleans them up.

Note: This does not remove lines from METALOG corresponding to these
packages, and makefs emits an error when it attempts to create the
filesystem but cannot find the files listed in METALOG -- but makefs
does seem to complete normally despite the error messages.

This change should be reverted once the pkg issue has been located and
fixed.

Approved by: re (cperciva)
Reviewed by: ivy
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53543

(cherry picked from commit 6a13aeac3c1f98db7cf156f24a4d6bc8d3c321f6)
(cherry picked from commit a21cff76c57089cd357473128d42410d189ca149)