bazel: Use jdk11_headless to build bazel

Use the same JDK for building bazel and for its runtime.

Effectively, the former `toolchain_hostjdk8` java toolchain has been deprecated
and should no longer be used (in newer bazel)[1]:

```
# Deprecated, do not use.
# It will be removed after migration to Java toolchain resolution.
default_java_toolchain(
name = "toolchain_hostjdk8",
...
)
```

[1]: https://github.com/bazelbuild/bazel/blob/4fc48680653a71aacbfd555436ba8f9a0742d3d9/tools/jdk/BUILD.tools#L384-L387

pkgs/development/tools/build-managers/bazel/bazel_3/default.nix

···
     [ bash coreutils findutils gawk gnugrep gnutar gnused gzip which unzip file zip ];
 
   # Java toolchain used for the build and tests
-  javaToolchain = "@bazel_tools//tools/jdk:toolchain_host${buildJdkName}";
+  javaToolchain = "@bazel_tools//tools/jdk:toolchain_${buildJdkName}";
 
   platforms = lib.platforms.linux ++ lib.platforms.darwin;
 

pkgs/top-level/all-packages.nix

···
   bazel_3 = callPackage ../development/tools/build-managers/bazel/bazel_3 {
     inherit (darwin) cctools;
     inherit (darwin.apple_sdk.frameworks) CoreFoundation CoreServices Foundation;
-    buildJdk = jdk8_headless;
-    buildJdkName = "jdk8";
+    buildJdk = jdk11_headless;
+    buildJdkName = "java11";
     runJdk = jdk11_headless;
     stdenv = if stdenv.cc.isClang then llvmPackages_6.stdenv else stdenv;
     bazel_self = bazel_3;