certificates can be passed through env vars

[arjunajesh]

What does this PR change? What problem does it solve?

This pull request enables the user to pass certificate files through environment variables (RESTIC_CACERT, RESTIC_TLS_CERT), instead of manually passing the file with --cacert or --tls-client-cert.

Was the change previously discussed in an issue or on the forum?

Yes it was discussed in an issue: #1926
Closes #1926

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[MichaelEischer]

Thanks for working on this issue. The PR still needs a few changes, I've added corresponding comments below. The new environment variables should also be added to the list in doc/040_backup.rst.

[MichaelEischer]

LGTM. I've rebased the PR and added some further small cleanups.