Musl builds broken on readonly filesystems

Compiling using the Nix package manager on `musl` panics:-

```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }', /nix/store/3bcpw5cb4pibbj8hc1z21m5i9rxa9s3r-crates-io/openssl-src-111.22.0+1.1.1q/src/lib.rs:547:26
```

This is because of

```rust
fs::write(path, buf).unwrap();
```

which tries to overwrite `crypto/rand/rand_unix.c`. Nix mounts `/nix/store` readonly.