Specify permissions for the GitHub Actions

[lslezak]

Problem

• The GitHub security scanner complains about missing permissions in the GitHub Actions
• Example: https://github.com/agama-project/agama/security/code-scanning/25

Solution

• Specify the permissions
• In the most cases the minimum contents: read is enough, that allows just reading the Git content and history
• The Actions which deal with translations needs push permissions and permissions for creating a new pull request

[coveralls]

coverage: 63.028% (-25.8%) from 88.872%
when pulling c193fcb on action_permissions
into 2e436fc on master.

[mvidner]

LGTM, thanks