limactl start: fix the issue where using --set would overwrite the existing instance's lima.yaml without validation.

[norio-nomura]

limactl start --set can overwrite the existing lima.yaml without validation, potentially creating an invalid lima.yaml.
e.g.

$ limactl create template://docker-rootful --name docker-test --tty=false
INFO[0000] Terminal is not available, proceeding without opening an editor 
INFO[0000] Attempting to download the image              arch=aarch64 digest="sha256:c841bac00925d3e6892d979798103a867931f255f28fefd9d5e07e3e22d0ef22" location="https://cloud-images.ubuntu.com/releases/24.04/release-20240423/ubuntu-24.04-server-cloudimg-arm64.img"
INFO[0000] Using cache "/Users/norio/Library/Caches/lima/download/by-url-sha256/b154bd45a2ac868f3d26ef79c803af364d0c7308eb72e602324cbf04cf9a8455/data" 
INFO[0000] Run `limactl start docker-test` to start the instance. 
$ limactl start docker-test --tty=false --set ".os = 1"
INFO[0000] Using the existing instance "docker-test"    
FATA[0000] errors inspecting instance: [field `os` must be "Linux"; got "1"] 
$ limactl start docker-test --tty=false
INFO[0000] Using the existing instance "docker-test"    
FATA[0000] errors inspecting instance: [field `os` must be "Linux"; got "1"] 

This PR changes the process to include validation before overwriting.

[AkihiroSuda]

Thanks

[norio-nomura]

Thanks! 🙏🏻