Release build script builds JVM components from wrong branch

### Describe the bug

## Description

The `build-release-comet.sh` script has a bug where it builds native binaries from the specified branch (via `-b` flag) but builds JVM components from whatever branch is currently checked out on the host machine. This can result in mismatched releases where native code and JVM code come from different branches.

## Steps to Reproduce

1. Check out `main` branch on host machine
2. Run the release build script with a different branch specified:
```bash
./build-release-comet.sh -b branch-0.13
```
3. Observe the resulting artifacts

## Expected Behavior

The entire build (both native binaries and JVM components) should be built from the branch specified via the `-b` flag (`branch-0.13` in this example).

## Actual Behavior

- Native binaries are correctly built from `branch-0.13` (inside Docker containers)
- JVM components and git hash are built/captured from `main` (on the host machine)

This creates a mismatched artifact that combines code from two different branches.

## Root Cause

The script has two separate build phases:

1. **Native binaries** (lines 132-160): Built inside Docker containers that properly clone the repo and checkout the specified branch
```bash
docker run ... $BUILDER_IMAGE_AMD64 "${REPO}" "${BRANCH}" amd64
```

2. **JVM components** (lines 196-209): Built on the host machine from whatever is currently checked out
```bash
pushd $COMET_HOME_DIR
GIT_HASH=$(git rev-parse --short HEAD) # Gets hash from current checkout!
./mvnw install # Builds from current checkout!
```

The script never switches branches on the host machine before building the JVM components.

## Proposed Solutions

### Option 1: Check out the branch on host (Recommended)
Add a git checkout before building JVM components:
```bash
pushd $COMET_HOME_DIR
git checkout "${BRANCH}" # Add this line
GIT_HASH=$(git rev-parse --short HEAD)
./mvnw install
popd
```

### Option 2: Document the requirement
If the script is intended to build JVM components from the current branch, document this clearly:
- Update the usage message to indicate the user must be on the correct branch
- Add a validation check that fails if the current branch doesn't match the `-b` parameter

### Option 3: Build everything in Docker
Build JVM components inside the Docker container alongside the native binaries to ensure consistency.

## Impact

This bug can lead to:
- Release artifacts that combine code from multiple branches
- Version mismatches between native and JVM components
- Difficult-to-debug issues when the release doesn't behave as expected
- Incorrect git hashes in release metadata

## Environment

- Script: `dev/release/build-release-comet.sh`
- Affected versions: Current main branch

### Steps to reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_