[BUG] Git is not using the configured SSH key when there is another SSH key added to the SSH agent

# Versions

- Git 2.30.1
- macOS 10.15.7

# Steps to Reproduce

- Have two SSH key pairs on your machine, one with a passphrase and
other without. The keys were created by running `ssh-keygen` in my
case.

- Add the key with a passphrase to the SSH agent by running `ssh-add`
and entering the passphrase.

- Have an entry like the following in your `~/.ssh/config`:

      Host someName
          HostName bitbucket.org
          User git
          IdentityFile ~/.ssh/private-key-of-the-pair-without-a-passphrase

- Add the public key of the pair without a passphrase to your BitBucket
account.

- Run `git clone someName:path/to/your/repository.git`. You will get a
not authorized error. As far as I can tell, this means that Git has
tried to use the key with the passphrase (which is added to the SSH
agent). Since this key is **not** the key that was added to the
BitBucket account, we received a not authorized error.

- Add the key without a passphrase to your SSH agent by running
`ssh-add` and try the same command (`git-clone`) again. It will work.

- Now, remove both keys from the SSH agent by running `ssh-add -D` and
try and try the same command (`git-clone`) again. It will still work.

I didn't test but don't believe this has something to do with the key having a passphrase or not. Also, I believe that I'm not using macOS Keychain to store the decrypted keys.

# Expected Behavior

The `git-clone` command to work as expected, regardless of which keys are present in the SSH agent, since the information that I provide to Git (via the `Host` information in the `~/.ssh/config` file) is sufficient.

# Actual Behavior

Git is trying to use the SSH key that is present in the SSH agent, instead of using the SSH key which is explicitly declared in the `~/.ssh/config` file.