Add support for specifying --host via environment variable

[maouw]

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

This PR adds the ability to set the hostname for most commands that accept the --host flag via the RESTIC_HOST environment variable. This allows the user to set the hostname once and not have to specify it for every command. Commands that accept --host but do not work on snapshots (e.g. restic key ...) are not affected by this change. The --host flag is still accepted and takes precedence over the RESTIC_HOST environment variable.

The changes are in cmd_backup.go for the backup command and find.go for commands that use the restic.SnapshotFilter options.

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

Closees #4733

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.

[maouw]

I have a question to the maintainers -- where and/or how would you suggest putting the proper tests for this feature? Most of the tests related to this area seem to circumvent option processing, so I can't find a good example to follow. I am a (relative) beginner to golang.

[MichaelEischer]

Thanks for the PR! I only have a few comments, see below.

The best way for testing is probably to use something like the following. That simply creates a new flagSet, configures it, then parses a test string and verifies the outcome:

set := pflag.NewFlagSet("test", pflag.PanicOnError)
flt := &restic.SnapshotFilter{}
initMultiSnapshotFilter(set, flt, false)
err := set.Parse([]string{"--host", "abc"})
rtest.OK(t, err)
// assert on flt.Hosts

[MichaelEischer]

LGTM. I've applied my suggestions and added a short test.

[maouw]

Thank you for taking care of the test. Very happy to see this coming through, it will be a great help.