mkcert silently overwrites CSR file

running mkcert 1.4.2.

when using the `-csr` flag, mkcert silently replaces the CSR file `www.example.com.pem` with a file of the same name `www.example.com.pem` that contains the signed certificate. instead of this destructive behaviour, mkcert could exit with an error instead of overwriting.

1. create a CSR file in an external system and place in the current directory
1. name the CSR file `www.example.com.pem`
1. run `mkcert -csr www.example.com.pem`
1. the file `www.example.com.pem` is replaced with the signed cert, and the CSR is destroyed