On NixOS we cannot use the pre-built `ijar` and `singlejar` tools as they are not
statically build and fail to find the dynamic linker.

Fixes #278

We were still on version 4.0.0 but already got version 5.5.1 in the resolved dependency graph.

We need to use at least version 6.1.0 since former versions of rules_java failed to declare a
dependency on rules_license which is now referenced.

This fixes CI errors for toolchains/java:
```
external/bazel_tools/tools/osx/xcode_configure.bzl:93:14: Invoking xcodebuild failed, developer dir: /Applications/Xcode_12.4.app/Contents/Developer ,return code 1, stderr: src/main/tools/process-wrapper-legacy.cc:80: "execvp(xcrun, ...)": No such file or directory
```

Bazel requires `libtool` for creating static archives on Darwin instead of the `ar` tool.