Filter spawn strategies by execution platform

[Silic0nS0ldier]

This PR implements part of the Execution Platform Scoped Spawn Strategies proposal.

It adds a new flag --allowed_strategies_by_exec_platform which permits filtering spawn strategies for spawns execution platform.

Example

# //.bazelrc
# Default strategies (order sensitive)
build --spawn_strategy=remote,worker,sandboxed,local

# Mnemonic targeted strategy override (order sensitive)
build --strategy=BAR=remote,sandboxed

# Host platform allowed strategies
build --allowed_strategies_by_exec_platform=@platforms//host:host=local,sandboxed,worker

# Remote platform allowed strategies
build --allowed_strategies_by_exec_platform=//:remote_platform=remote

For an action with mnemonic FOO configured for the host platform (@platforms//host:host), it will resolve worker,sandboxed,local as it's spawn strategy candidates.

• remote was eliminated as a candidate (not in allow list for platform).
• Order from --spawn_strategy was preserved.

For an action with mnemonic BAR configured for the host platform (@platforms//host:host), it will resolve sandboxed as it's spawn strategy candidate.

• remote was eliminated as a candidate (not in allow list for platform).
• Mnemonic override applied, leaving sandboxed as the final candidate.

For an action with mnemonic BAR configured for the remote platform (//:remote_platform), it will resolve remote as it's spawn strategy candidate.

• sandboxed was eliminated as a candidate (not in allow list for platform).
• Mnemonic override applied, leaving remote as the final candidate.

If no spawn strategy candidate remains after filtering, the standard error will be logged.

ERROR: /workspaces/___/BUILD.bazel:3:22: _description_ [for tool] failed: _mnemonic_ spawn cannot be executed with any of the available strategies: []. Your --spawn_strategy, --genrule_strategy and/or --strategy flags are probably too strict. Visit https://github.com/bazelbuild/bazel/issues/7480 for advice

TODO

• Update "spawn cannot be executed" error to include new strategy flag.
• CoverageReportActionBuilder.CoverageReportAction action uses spawn machinery but lacks an execution platform, leading to a precondition check failure. Being resolved in Give builtin actions a platform #23231

[Silic0nS0ldier]

cc @katre