install: fix simplify conditional expressions and improve error handling

[mattsu2020]

Refactor several conditional blocks to use let-else statements for cleaner code flow. Simplify error handling in file removal operations by combining conditions. Update backup file handling to use more concise conditional logic. These changes improve code readability while maintaining the same functionality.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/install/install-C-root. tests/install/install-C-root is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!

[xtqqczze]

It looks like these changes simply fix a few instances of clippy::collapsible_if.

[xtqqczze]

I think this is a change in behavior

[cakebaker]

Good catch!

Yes, you are right, it's a change in behavior. The current implementation never returns true if group_id == to_meta.gid() whereas the proposed change might return true in such a case (depending on what needs_copy_for_ownership(to, &to_meta) returns).

[xtqqczze]

I think we can't make any assumptions of correctness with AI generated PRs; we have to verify everything. In particular changes that "simplify conditional expressions" are high-risk.

[xtqqczze]

@cakebaker In this case there actually are conditions that can be made much more clear, I've submitted #11280.

[xtqqczze]

It looks like these changes simply fix a few instances of clippy::collapsible_if.

Since the remaining changes are purely clippy::collapsible_if fixes, I’ve submitted #11285 with a description of how the fixes were applied, without any AI involvement.