v3.0: fix: Show correct default path in signer error messages (backport of #7849)

[mergify]

Problem

When Solana CLI configuration contains an invalid keypair_path, error messages suggest creating new keypairs at the misconfigured location instead of the user's actual default directory. This leads to confusion and keypairs being created in wrong locations (e.g., /home/sol/identity/id.json instead of $HOME/.config/solana/id.json).

Solution

Modified error message generation in DefaultSigner::path() to always suggest the system default path (~/.config/solana/id.json) based on the user's actual home directory, regardless of any misconfigured paths in config files.

@KirillLykov can you check this, closes #6637

---

This is an automatic backport of pull request #7849 done by Mergify.

[mergify]

Cherry-pick of 7c3e9e7 has failed:

On branch mergify/bp/v3.0/pr-7849
Your branch is up to date with 'origin/v3.0'.

You are currently cherry-picking commit 7c3e9e745.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Cargo.lock
	modified:   clap-utils/Cargo.toml
	modified:   clap-v3-utils/Cargo.toml
	modified:   programs/sbf/Cargo.lock

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   clap-utils/src/keypair.rs
	both modified:   clap-v3-utils/src/keypair.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[t-nelson]

i think this one is wrong too #7849 (comment)

also merge conflicts need resolved