Allow kernel set with tar to use relative paths to tar file (#582)

## Type of Change
- [x] Bug fix

## Description
Allow kernel set with tar to use relative paths to tar file. Fixes
https://github.com/apple/container/issues/573.

## Motivation and Context
`absoluteString` will prefix a scheme to the file path that looks like
"file://". This will cause file manager to fail to find the file at the
file path even if it exists. Change to instead just get the `path` of
the file, which does not add a scheme prefix.

## Testing
- [x] Tested locally

Signed-off-by: Kathryn Baldauf <k_baldauf@apple.com>

Sources/CLI/System/Kernel/KernelSet.swift

@@ -77,7 +77,7 @@ extension Application {
                 throw ArgumentParser.ValidationError("Missing argument '--tar")
             }
             let platform = try getSystemPlatform()
-            let localTarPath = URL(fileURLWithPath: tarPath, relativeTo: .currentDirectory()).absoluteString
+            let localTarPath = URL(fileURLWithPath: tarPath, relativeTo: .currentDirectory()).path
             let fm = FileManager.default
             if fm.fileExists(atPath: localTarPath) {
                 try await ClientKernel.installKernelFromTar(tarFile: localTarPath, kernelFilePath: binaryPath, platform: platform, force: force)