[rubygems/rubygems] Fix misleading error if compact index cannot be copied

Previously if `~/.bundle/cache/compact_index/rubygems.org.*/version`
were owned by root with read-only access, `bundle install` would fail
with a misleading error message. For example:

```
There was an error while trying to write to `/tmp/bundler-compact-index-20220711-1823-npllre/versions`. It is
likely that you need to grant write permissions for that path.
```

This happened because the EACCESS error was caught by
`SharedHelpers.filesystem_access`, which makes it look like the target
directory is at fault instead of the source.

We can't simply drop this guard because that causes the opposite
problem: the permission error appears to come from the source instead of
the target, since `CompactIndexClient::Cache#lines` also wraps read
access errors.

Instead, bring a minimal implementation of `FileUtils.cp` and nest calls
to `SharedHelpers.filesystem_access` properly.

https://github.com/rubygems/rubygems/commit/320822c070

Co-authored-by: Stan Hu <stanhu@gmail.com>

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