add --extra_toolchains flag to bzlmod example

[dan-stowell]

To get bazel build //:main --config=remote-linux working for examples/bzlmod, I had to add the --extra_toolchains flag. I am on OS X building remotely on Linux.

(I would like to test builds with the toolchain in preparation for a release pipeline.)

[sluongng]

I think we should add this in the commented-out form, not as-is.

The reason is that BzlMod automatically registers the toolchain for you based on the content of the bazel_dep's MODULE.bazel file. That makes the flag --extra_toolchains= redundant.

For example, let's say I have a module toolchains_foo with @@toolchains_foo//:MODULE.bazel file calling register_toolchains(...). Those toolchains will then be available automatically for selection when the user declares bazel_dep(name = "toolchains_foo") in their project's MODULE.bazel file.

However, there are cases where a user can have multiple toolchains of the same type automatically registered. In those cases, the --extra_toolchains flag does come in handy as it manually selects the exact toolchain, avoiding any error in the automatic selection process.

[dan-stowell]

I think we should add this in the commented-out form, not as-is.

The reason is that BzlMod automatically registers the toolchain for you based on the content of the bazel_dep's MODULE.bazel file. That makes the flag --extra_toolchains= redundant.

I'm on a MacBook Pro trying to get the bzlmod example to build using RBE. If I do not pass --extra_toolchains, the build does not succeed.

dan@dan-bb-mbp bzlmod % bazel --nohome_rc build --config=remote-linux //:main
INFO: Invocation ID: 8690133f-4de6-4bad-9fc5-51e7928569df
INFO: Streaming build results to: https://app.buildbuddy.io/invocation/8690133f-4de6-4bad-9fc5-51e7928569df
ERROR: /private/var/tmp/_bazel_dan/b759800e61deba813376bced2cb79625/external/rules_cc+/cc/BUILD:103:19: in cc_toolchain_alias rule @@rules_cc+//cc:current_cc_toolchain:
Traceback (most recent call last):
        File "/virtual_builtins_bzl/common/cc/cc_toolchain_alias.bzl", line 26, column 48, in _impl
        File "/virtual_builtins_bzl/common/cc/cc_helper.bzl", line 165, column 13, in _find_cpp_toolchain
Error in fail: Unable to find a CC toolchain using toolchain resolution. Target: @@rules_cc+//cc:current_cc_toolchain, Platform: @@toolchains_buildbuddy+//platforms:linux_x86_64, Exec platform: @@bazel_tools//tools:host_platform
ERROR: /private/var/tmp/_bazel_dan/b759800e61deba813376bced2cb79625/external/rules_cc+/cc/BUILD:103:19: Analysis of target '@@rules_cc+//cc:current_cc_toolchain' (config: e09d445) failed
ERROR: Analysis of target '//:main' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.442s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully
INFO: Streaming build results to: https://app.buildbuddy.io/invocation/8690133f-4de6-4bad-9fc5-51e7928569df
dan@dan-bb-mbp bzlmod % bazel --nohome_rc build --config=remote-linux //:main --extra_toolchains=@toolchains_buildbuddy//toolchains/cc:ubuntu_gcc_x86_64
INFO: Invocation ID: f32d267e-19dd-4d9e-a090-d3d7125aa705
INFO: Streaming build results to: https://app.buildbuddy.io/invocation/f32d267e-19dd-4d9e-a090-d3d7125aa705
WARNING: Build option --extra_toolchains has changed, discarding analysis cache (this can be expensive, see https://bazel.build/advanced/performance/iteration-speed).
INFO: Analyzed target //:main (4 packages loaded, 458 targets configured).
INFO: Found 1 target...
Target //:main up-to-date:
  bazel-bin/main
INFO: Elapsed time: 0.288s, Critical Path: 0.08s
INFO: 2 processes: 5 action cache hit, 1 remote cache hit, 1 internal.
INFO: Build completed successfully, 2 total actions
INFO: Streaming build results to: https://app.buildbuddy.io/invocation/f32d267e-19dd-4d9e-a090-d3d7125aa705

[dan-stowell]

We're tinkering with #53 instead