`az functionapp keys set` throws 'Not Found'

## Describe the bug

Possibly related issue: Azure/Azure-Functions#2221

**Command Name**
`az functionapp keys set`

**Errors:**
```
Operation returned an invalid status 'Not Found'
```

`az functionapp show --name {} --resource-group {}` with the same parameter values works as expected.

## To Reproduce:
- Linux Function App v4.
- Configured to pull container image from Azure Container Registry using managed identity.
- `az functionapp keys set --key-type masterKey --name {} --resource-group {} --key-name "_master"`

## Expected Behavior
A key named `_master` with a generated value was created and assigned to the function app.

## Environment Summary
```
Linux-4.4.0-19041-Microsoft-x86_64-with-debian-bullseye-sid, Ubuntu 20.04 LTS
Python 3.6.10
Installer: DEB

azure-cli 2.31.0

Extensions:
aks-preview 0.5.49
```

## Additional Context

Start by reproducing `az functionapp keys set` on a Linux Function App v4 using the container and managed-identity setup described, then compare it with the working `az functionapp show` command. Read the related Azure/Azure-Functions#2221 issue for context. Done means the `_master` key is created with a generated value instead of returning Not Found.

Written by the indexing model from the issue text.