Allow the user to override the commands for git, ssh-agent, and ssh-add (#154)

On my self-hosted Windows runners, the `git`, `ssh-agent`, and `ssh-add`
commands are not located in the locations that are currently hard-coded
in `paths.js`.

With this PR, I am able to get this action to work on my runners as
follows:

```yaml
- uses: webfactory/ssh-agent@...
  with:
    ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
    git-cmd: git
    ssh-agent-cmd: ssh-agent
    ssh-add-cmd: ssh-add
```

action.yml

@@ -10,6 +10,15 @@ inputs:
         description: 'Whether or not to log public key fingerprints'
         required: false
         default: true
+    ssh-agent-cmd:
+        description: 'ssh-agent command'
+        required: false
+    ssh-add-cmd:
+        description: 'ssh-add command'
+        required: false
+    git-cmd:
+        description: 'git command'
+        required: false
 runs:
     using: 'node16'
     main: 'dist/index.js'