bug: resolve keyring flag properly

[dudong2]

Description

ref. cosmos/cosmos-sdk#19060

how to test

• test command cantod tx feegrant prune --from <addr> --gas-prices 10000000000000000acanto --home <home dir> --chain-id canto_7700-1 --keyring-backend test. If no error occured, it is fixed well.

Closes: #87

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• included the necessary unit and integration tests
• reviewed "Files changed" and left comments if necessary

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[dongsam]

The review for this PR has been completed in in b-harvest#88 and b-harvest#89, and since it was a simple cli level bug fix and docs update without state logical modification, I will proceed with the merge to meet the v8 release schedule.