tool_util.deps: please support setting the platform for conda via dependency manager config key

[mr-c]

In cwltool we have a test to use conda (including the bioconda channel) to install seqtk via galaxy.tool_util.deps.build_dependecy_manager.

Recently the default macos-latest GitHub Actions hosted runner switched to an M1 Apple Silicon (osx-arm64) system, which causes our test to fail as bioconda is not yet making osx-arm64 builds.

However, Apple Silicon users have Rosetta, and can install/run osx-64 binaries if a osx-64 platform conda is installed and configured with --platform osx-64

The platform mismatch could also happen to a user; this is not just a CI issue.

It would be nice if there was a conda_platform option we could set to osx-arm64 in the app_config_dict when calling galaxy.tool_util.deps.build_dependecy_manager

Would the correct procedure be as follows?

1. Add a platform optional keyword parameter to CondaContext.__init__ and save it as an instance variable.
2. Update CondaContext.exec_create(), to consult self.platform; adding f"--platform={self.platform}" to the args if self.platform is not None.
3. Update galaxy.tool_util.deps.resolvers.conda.CondaDependencyResolver.__init__() where the CondaContext instantiation happens and set platform=get_option("platform"),
4. Mention this in doc/source/admin/dependency_resolvers.rst

Note: in 3, we would look up platform as the key instead of conda_platform, as get_resolver_option automatically prefixes the name of the resolver (conda in our case) plus an underscore.

[nsoranzo]

Looks like a great plan to me, thanks @mr-c !