packaging: automated COPR builds on release

[AldanTanneo]

Fixes #6469

This is a possible way of setting up COPR builds for jj, but it would need setting up an official jj account on https://copr.fedorainfracloud.org first.

On each release this would write the version (from the tag name) to the copr.spec file and schedule the build on COPR from it. It does not create packages for every dependency, instead it does an online build. No sources are bundled, the SRPM is created from the release tarball by the COPR infrastructure. This is how I set up my own COPR (https://github.com/AldanTanneo/copr-jj), but the Github action there has to check for new releases every day, whereas this action can just react to new releases.

Needed for working builds:

• a COPR account named jujutsu (tbd)
• a project in this account named jj-cli (tbd)
• credentials for this account added to Github actions secrets: COPR_LOGIN, COPR_TOKEN.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes

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