gRPC variable usage

### Expected Behavior

Able to use variables in the nav bar.

### Actual Behavior

Unable to use variables as part of the nav bar.

Work around is to specify the variable in the request body and copy/paste it into the nav bar.

### Reproduction Steps

1. Create a gRPC method
2. Try to specify a variable - completion never occurs

### Is there an existing issue for this?

- [X] I have searched the [issue tracker](https://www.github.com/Kong/insomnia/issues) for this problem.

### Additional Information

_No response_

### Insomnia Version

2021.7.2

### What operating system are you using?

macOS

### Operating System Version

macOS big sur

### Installation method

Downloaded from insomnia.rest

### Last Known Working Insomnia version

_No response_

Reproduce the issue by creating a gRPC method and trying to specify a variable in the nav bar, then compare that behavior with copying a variable from the request body. Trace the gRPC nav-bar completion flow and consider the issue complete when variables can be specified there and completion works as expected.

Written by the indexing model from the issue text.