Where to install musl. A good choice is something like $HOME/musl.
This is where the musl-gcc wrapper gets installed to. It should point to somewhere in your PATH. A good choice is something like $HOME/bin.
This is where the dynamic linker gets installed into. The default is /lib; this path will be baked into binaries built by musl-gcc, so you should not alter it if you want your dynamic binaries to be portable. Installing to /lib on most systems requires root privileges, so a typical choice for non-root users would be $HOME/musl/lib.