0.21 removed the bundled openjdk-distribution. Instead, tries to fetch
the “right” distribution on-the-fly when building.
So we need to provide our own openjdk.
According to
https://github.com/bazelbuild/bazel/issues/6865#issuecomment-447261288
we should set `--host_javabase="@local_jdk//:jdk` if we want to do
that. This uses the jdk that is currently in the environment, which is
openjdk 8 in our case. 0.21 defaulted to a toolchain for JDK9, which
we don’t package in nixpkgs, so we use the JDK8 toolchain.
This commit also replaces the line-number-based sed invocations with
something more stable.
9e9fec640e