* fixed new gocritic violations
* fixed new 'contextcheck' violations
* fixed 'gosec' warnings
* suppressed ireturn and varnamelen linters
* fixed tenv violations, enabled building robustness tests on arm64
* fixed remaining linux failures
* makefile: fixed 'lint-all' target when running on arm64
* linter: increase deadline
* disable nilnil linter - to be enabled in separate PR
* content: fixed repo upgrade version
Previously upgrade would enable epoch manager and index v2 but would
not set the version of the format itself. Everything worked fine
but it would not protect from old kopia opening the repository.
* ci: added compatibility test that uses real 0.8 and current binaries
Instead of using hardcore Makefile scripts and external tools, created a
specialized Go tool that combines curl, sha256sum, gunzip, tar, unzip
and automatically figures out the correct URL to download based on
the current GOOS/GOARCH combination.
