gettext: Use a git credential helper to set token

[davidmhewitt]

The GitHub checkout@v2 action sets the credentials on the repo differently, meaning that doing this does not work anymore:

actions/gettext-template/entrypoint.sh

Line 35 in 4e4476d

git remote set-url origin https://x-access-token:"$GITHUB_TOKEN"@github.com/"$GITHUB_REPOSITORY".git

This means that the action can no longer push to protected branches using the elementaryBot token.

Reading this suggests that adding a git credential helper instead should be the solution to this problem:
actions/checkout#162

So it looks like we can add something like this instead:

git config credential.https://github.com/.helper "! f() { echo username=x-access-token; echo password=$GITHUB_TOKEN; };f"