integ-runner: needs to read feature flags from target repo, not its own built-in set

### Describe the bug

Otherwise the flags are subject to unexpected change, which impedes upgrading.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

Snapshots don't break during upgrade.

### Current Behavior

Snapshots break during upgrade.

### Reproduction Steps

--

### Possible Solution

Read the set of flags from a file, write that file with the recommended set if it doesn't exist yet.

### Additional Information/Context

_No response_

### CDK CLI Version

-

### Framework Version

_No response_

### Node.js Version

-

### OS

-

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_