Include task-specific shell commands and build flags

[joeleba]

Description of the problem / feature request:

For example: https://github.com/bazelbuild/bazel/blob/master/.bazelci/postsubmit.yml#L119

ubuntu1804_javabase9:
    platform: ubuntu1804_nojava
    name: ":java: OpenJDK 9 Javabase"
    shell_commands:
      - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
        android_ndk_repository/android_ndk_repository/' WORKSPACE
      - rm -f WORKSPACE.bak
    build_flags:
      - "--javabase=@openjdk9_linux_archive//:runtime"
...
ubuntu1804_javabase10:
    platform: ubuntu1804_nojava
    name: ":java: OpenJDK 10 Javabase"
    shell_commands:
      - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
        android_ndk_repository/android_ndk_repository/' WORKSPACE
      - rm -f WORKSPACE.bak
    build_flags:
      - "--javabase=@openjdk10_linux_archive//:runtime"

The same ubuntu1804_nojava platform can have many tasks based on it, each with a different set of shell_commands and build_flags.

It's useful to perform benchmarking on each of these different environments.