I recommend using choosenim. I've installed nim via brew in the past, but choosenim is more comprehensive, ex:
» choosenim show
Selected: 0.20.2
Channel: stable
Path: ~/.choosenim/toolchains/nim-0.20.2
» choosenim versions
Channel: stable
Installed:
* 0.20.2 (latest)
Available:
0.20.0
0.19.6
0.19.4
0.19.2
0.19.0
0.18.0
0.17.2
0.17.0
0.16.0
Can't really get that with brew.