I don't know if there is a solution to use a break wherever you want, but playing with css will do the job for sure.
library(shiny)
shinyApp(
ui = fluidPage(
actionButton("btnId", "I want a line break here<br/>since the label is too long",
style="white-space: normal;
text-align:center;
color: #fff;
background-color:#CC3300;
border-color: #2e6da4;
height:50px;
width:150px;
font-size: 10px;")),
server = function(input, output){})