use_lintr() shouldn't error on existing config

I foud myself trying `use_lintr()` on a package with an existing `.lintr` file because I wanted the other side effect(s), currently just fixing up the .Rbuildignore, but after #2994 and beyond, others as well. However currently it immediately fails: ```r use_lintr() # Error in `use_lintr()`: # ! Found an existing configuration file at /.../.lintr. ```