[electrs] Build fails due to `librocksdb-sys` build command

Unfortunately building `Blockstream/electrs` currently fails due to:

```
error: failed to run custom build command for `librocksdb-sys v6.20.3`

Caused by:
process didn't exit successfully: `/home/ero/workspace/electrs/target/debug/build/librocksdb-sys-fe4c5565d1d2e4df/build-script-build` (exit status: 101)
--- stderr
thread 'main' panicked at 'Unable to find libclang: "couldn't find any valid shared libraries matching: ['libclang.so', 'libclang-*.so', 'libclang.so.*', 'libclang-*.so.*'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"', /home/ero/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bindgen-0.59.2/src/lib.rs:2144:31
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
```

Tested on macOS and Linux with a number of different Rust versions.