dev: use cmd.Context inside custom command

[alexandear]

• Use cmd.Context() instead of context.Background. Currently, they are the same but this might change in the future.
• Remove the space from the custom command short description for consistency with other commands in this repo.
• Capitalize the stderr output message and add a semicolon for clarity:
  • before:

    > ./custom-golangci-lint run
    ...
    failed executing command with error build linters: plugin(someplugin): plugin "someplugin" not found
    

  • after:

    > ./custom-golangci-lint run
    ...
    Failed executing command with error: build linters: plugin(someplugin): plugin "someplugin" not found