guix package: 'transaction-upgrade-entry' swallows build requests.

guix package: 'transaction-upgrade-entry' swallows build requests.

Fixes a regression introduced in
131f50cdc9dbb7183023f4dae759876a9e700bef whereby the install/upgrade
message would not be displayed:

  $ guix upgrade -n
  2.1 MB would be downloaded:
     /gnu/store/…-something-1.2
     /gnu/store/…-its-dependency-2.3

This is because we'd directly abort from 'transaction-upgrade-entry' to
the build handler of 'build-notifier'.

* guix/scripts/package.scm (transaction-upgrade-entry): Call 'string=?'
expression in 'with-build-handler'.
* tests/packages.scm ("transaction-upgrade-entry, grafts"): New test.