Use the launcher_maker toolchain if available

[fmeum]

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[rickeylev]

fwiw/fyi: we have a CI job that uses bazel-rolling, in case that's helpful.

Also, a fun trick is using FeatureFlagInfo to detect if a toolchain is present. Something like:

def impl(ctx):
  return FeatureFlagInfo(value=str(ctx.toolchains["foo"] == None))
has_tc = rule(
  implementation = impl,
  toolchains = [toolchain_type("foo", mandatory=False)]

has_tc(name="has_tc")
config_setting("is_has_tc_true", flag_values = {":has_tc": "True"})

alias("launcher_maker", actual = select({
  "is_has_tc_true": ...,
  })
)

[fmeum]

Adding a dep on bazel_features isn't easy with rules_python's WORKSPACE setup since it doesn't use two levels of macros. Can we add a call to bazel_features_deps to the relnotes?