fix: `pull --rebase` before `push`

[buschtoens]

When commits are pushed to CI in quick succession, the deployment can fail with this error:

+- upload
|  |
|  +- git
To ember-intl/ember-intl.git
 ! [rejected]          gh-pages -> gh-pages (fetch first)
error: failed to push some refs to 'ember-intl/ember-intl.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
|
+- didFail
Error: git push ember-intl/ember-intl.git gh-pages exited with nonzero status
Error: git push ember-intl/ember-intl.git gh-pages exited with nonzero status
    at ChildProcess.<anonymous> (/home/runner/work/ember-intl/ember-intl/node_modules/ember-cli-deploy-git/lib/run.js:17:19)
    at ChildProcess.emit (events.js:315:20)
    at ChildProcess.EventEmitter.emit (domain.js:486:12)
    at maybeClose (internal/child_process.js:1048:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:288:5)
|
Pipeline aborted

https://github.com/ember-intl/ember-intl/runs/1571595497

The reason is that the prior CI run is pushing a new commit, while the current CI run is building. Once the current CI run then tries to push, the remote rejects, as the new commit from the prior run is missing.

By running git pull --rebase before git push, we can avoid this.

(Disclaimer: I haven't tested this yet, but can give it a shot later.)

[ef4]

This seems good if you can get CI green and confirm that it's tested.