CI: remove (future) deprecated --preserve-env flag

- explicitly pass GEM_HOME

Fixes:
```
sudo: preserving the entire environment is not supported, '--preserve-env' is ignored
ubuntu@21f652d6-b208-4386-b0c0-097e26578228:/tmp/build/44575cf5$ set -e
ubuntu@21f652d6-b208-4386-b0c0-097e26578228:/tmp/build/44575cf5$
ubuntu@21f652d6-b208-4386-b0c0-097e26578228:/tmp/build/44575cf5$ cd "/tmp/build/44575cf5/bosh-linux-stemcell-builder"
ubuntu@21f652d6-b208-4386-b0c0-097e26578228:/tmp/build/44575cf5/bosh-linux-stemcell-builder$ bundle install
Bundler 4.0.13 is running, but your lockfile was generated with 2.5.23. Installing Bundler 2.5.23 and restarting using that version.
Fetching gem metadata from https://rubygems.org/.
Fetching bundler 2.5.23

Retrying download gem from https://rubygems.org/ due to error (2/4): Bundler::PermissionError There was an error while trying to write to `/usr/local/lib/ruby/gems/3.3.0/cache/bundler-2.5.23.gem`. It is likely that you need to grant write permissions for that path.
```
^ https://bosh.ci.cloudfoundry.org/teams/stemcell/pipelines/ubuntu-resolute-builder/jobs/build-os-image/builds/1#L6a052874:4:13

Author: aramprice

ci/tasks/build.sh

@@ -77,7 +77,7 @@ chown -R ubuntu:ubuntu "${REPO_PARENT}/bosh-linux-stemcell-builder"
 chown -R ubuntu:ubuntu /mnt
 sudo chmod u+s "$(which sudo)"
 
-sudo --preserve-env --set-home --user ubuntu -- /bin/bash --login -i <<SUDO
+sudo --set-home --user ubuntu GEM_HOME="${GEM_HOME}" -- /bin/bash --login -i <<SUDO
 set -e
 
 cd "${REPO_PARENT}/bosh-linux-stemcell-builder"