ci: Fix again build-previous-qemu

The build-previous-qemu job is now trying to fetch from the upstream
repository, but the tag is only fetched into FETCH_HEAD:

$ git remote add upstream https://gitlab.com/qemu-project/qemu 00:00
$ git fetch upstream $QEMU_PREV_VERSION 00:02
warning: redirecting to https://gitlab.com/qemu-project/qemu.git/
From https://gitlab.com/qemu-project/qemu
 * tag                     v8.2.0     -> FETCH_HEAD
$ git checkout $QEMU_PREV_VERSION 00:02
error: pathspec v8.2.0 did not match any file(s) known to git

Fix by fetching the tag into the checkout itself.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

.gitlab-ci.d/buildtest.yml

@@ -190,7 +190,7 @@ build-previous-qemu:
   before_script:
     - export QEMU_PREV_VERSION="$(sed 's/\([0-9.]*\)\.[0-9]*/v\1.0/' VERSION)"
     - git remote add upstream https://gitlab.com/qemu-project/qemu
-    - git fetch upstream $QEMU_PREV_VERSION
+    - git fetch upstream refs/tags/$QEMU_PREV_VERSION:refs/tags/$QEMU_PREV_VERSION
     - git checkout $QEMU_PREV_VERSION
   after_script:
     - mv build build-previous