SSM plugin auto-installation is not supported on Windows

[rclinard-amzn]

Related to https://github.com/aws/copilot-cli/issues/2561 in the broader picture of the current SSM plugin auto-installation setup focusing on covering the common cases instead of all cases.

It looks like the code fails to even recognize the error and falls through to the default case on Windows: https://github.com/aws/copilot-cli/blob/f155f94a4db40d1917b647567cc0ca8d755c26c8/internal/pkg/cli/svc_exec.go#L268-L274

$ .\copilot.exe svc exec --name service --env dev -c /bin/sh
X Failed to validate the Session Manager plugin. Please install or make sure your SSM plugin is up-to-date:
https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager-working-with-install-plugin.html
X validate ssm plugin: exec: "session-manager-plugin": executable file not found in %PATH%

The feedback is still helpful, but if auto-installation were possible it would make things just a bit nicer to use on Windows.